I was affraid that this will be just another "evil bad" topic ...
But you have some great ideas ... personaly i would dare to presume that some of them are allready planed, since if you choose Bounty Hunter specialisation for your Ranger, there is specificly told that enemies restrained by this character will have it harder to escape ... yet there is no restraining option in game so far.
But there is a few things i like to coment, if you dont mind.
(Actualy i will coment them even if you do, but i would like to talk about it. :P )
d) FIght is ending, with Minthara having enemies close to her and she is brought to low health. She is asking for mercy and surrenders to us.
I cant imagine Minthara beging for mercy ...
If you think about it, she is Drow, Lolthsworn Drow, that was probably growing in Menzoberranzan ... that, and all the threatments prisoners get in goblin camp ... should be motivation enough to rather commit suicide, than risk falling into captivity. :-/ So more than that i can imagine her swalowing some poison.
Ofcourse we should be able to ressurect her and capture her anyway, it would be hilarious as hell.

g) Minthara dies and fight becomes easier, since the bad guys have no leader now
Goblins certainy should start runing, or surendering at this point ...
Maybe im wrong here, but they seem to me like fear of Minthara was the only thing that was driving them on. :-/
During the dialogues we should be able to treath her nicely, neutrally or badly, with each type of treatment affecting how she respond to us (like if we treat her well, she willingly reveals to us the information about Moonrise towers + Absolute and offers to join us)
I dont like this to be honest.
Yes we should completely be able to treath her nicely i agree ... but she should not willigly reveal us the informations, nor offer to join us, just bcs we are nice to her. :-/ That seem out of character.
If anything, we should be able to torture our answers out of her, and even in that case she should not give them easily.
But i just cant shake the feeling that Minthara is that type that will mock you for being soft, weak, patetic, and despicable if you treath her nicely.
I can imagine that this could work, but it would require a much longer work on her ... like treath her nicely since capturing, through moonrise towers where we show her what she actualy signed to, shaken her believes, and then help her deal with that ... and even that seem a little too fast for us to gain her thrust, after all we just broke her whole world.

But concidering it as long therm, it could work.
- Trying to convince her to join us (after all, she doesn't want to get punished for failing by her superiors, she needs protection from her previous "allies", only possible with good treatment, logical beginning for redemption arc, which given her character is something that is pretty possible, with later option for romance if player wishes so)
Same comentary as abowe ... yes, i can see this as possibility, and even fiting the character ... just not so fast.
After all, even David Hasselhoff wasnt able to save everyone.

a) If we ask Zevlor to move the tieflings out of the grove, he attacks us. After this happens (and he dies/surrenders/is knocked out), we should have the possibility of our party convincing tieflings to leave, with only druids remaining in the grove.
And here is the problem ... to leave where exactly?
I could imagine convincing them (with extremely hard persuation check) to repair boat and try their luck on water ... but that seem like the only option. :-/
The willage just across the first bridge is swarming with goblins and if you go the other way, you just reach the beach and its over. Also Tieflings are draging all their property with them, wagons with food, clothing and stuff ... not to mention civilians and childern. There is litteraly no way they could sneak around that goblin village.
Also note that bridge that is conecting Blighted Village to northern part of map is broken, so they even cant cross it with all their posessions.
The only another way i could imagine is persuating leader of Flaming Fist to provide them escort, once you help them save that elf from burning house.
I imagine that would also require some persuation check, since they have allready urgent business to do ... but its only fair to ask for favor, after you save her life.
b) With hard persuasion check, it should be possible that we can convince Minthara to only focus on druids, with her forces still looting the camp, but letting the tielfings go to spread the word about the Absolute or just by telling her that they might be useful later in future
Im not quite sure if that is even possible.
All we get so far is that goblins are searching some "weapon" in the groove ... no idea who, or what it should be.
But i presumed that Minthara was hunting for Tieflings at first, and finding druids is simply sideffect of her hunt.
So if that is the case, i cant imagine any way that she would let go her prey ...
On the other hand, in some other topics at this theme, i allready mentioned that i would like to see "some" Tieflings not being killed during attack.
Personaly i would like to see groove wiped out, and every armed Tiefling dead ... then civilians with kids being driven into a corner ... either in prison, or "dragon cave", or in Zevlor's cave, w/e it dont matter so much in the end ... and then Minthara let us decide their faith:
- Sell them to slavery ... either to Zhentarim, or to Underdark ... with conciderable cut for us, ofcourse.
- Sacrifice them for the absolute ... either right there, or send them to Moonrise Towers ...
- Give them to goblins, to kill/torture/eat.
- Execute them all at once personaly.
- Let them go, to spread word of what happened here ... with possibility to send goblins after them.
c) We let tieflings hide or escape during/before the siege through the tunnels
Simmilar problem as abowe (in persuating them to leave), they dont have anywhere to go as long as roads are swarming with goblins.
An example of persuasion could be, that we sided with "bad guys" because:
...
d) This should be only possible if the tieflings don't die
I dont see any conection between Halsin and Tieflings ...
So why this condition? O_o
b) Possibility of causing an interal war between the leaders, supporting one of them
This seem odd ...
Im not saing impossible, but certainly odd ... they all have their own parts of goblin army to comand, and they dont influence others at all ... their minds are even conected when close to each other. :-/
I cant see any way to make this work, that dont seem odd.
But there certainy should be more options of interacting with goblin leaders ...
Personaly i find quite anoying that you arent able to convince any other leader to join your attack on the Groove, at least with Ragzlin it should be quite easy.
c) Ability to become the leader of goblins yourself (after killing/capturing/"convincing" the rest)
I litteraly dont like this idea.
I agree it may sound awesome, but also cant shake the feeling that if would bring more problems than benefits.
How would you like to continue any story as goblin leader? :-/
f) Tricking the drow warlord into attacking false spot, again giving tieflings time to prepare
This again, would cause more harm than good ... if you study your map carefully, you find out that in this zone is actualy just a few places ...
And concidering that goblins allready raided most of it, the only place where you still can send them is possibly Githyanki chreche, Toll house, Hag's swamp, and Druid groove ... whole rest of the map they allready control.
(Its kinda funny that Minthara is unable to pinpoint goove location just concidering this, there isnt so much other options. But i gues that antagonist just need to be kinda stupid. :D)
In last patch there was added deception check roll to tell minthara false spot of druid groove location, and if you sucess she only tells you that you fall for prisoners lies, since they allready raided that place.
And personaly i find it perfectly fiting whole story.
g) Causing an internal war in Druid Grove, like described above (Tieflings vs Druids), to weaken the grove
You allready can, there is even several options for that.
- when you decide to side with Zevlor, and kill Kagha you need to kill all druids, wich makes sence since even if not all druids agree with Kaghas methods and leadership ... they probably wont just look aside when some strainger attact one of them.

- when you decide to side with Kagha, and fore Zevlor out (wich leads to killing him), after next long rest druids simply finish their ritual and seal the groove (and yes that means killing every single Tiefling).
- you can also find proof that Kagha is betraying druids, and confront her ... wich can also leads to two outcomes, where on one hand is Kagha and shadow druids dead ... and on the other one only shadow druids dead and Kagha sory for all her misstakes canceling the ritual and providing tieflings all the help they may need.
This options seem quite suficient to me, what would you like to add? O_o
h) Tell defenders fake information about how the goblins will attack, to make defense harder and make it impossible for them to prepare.
Well, that would require complete rework of how it works now.
And i quite like that Tieflings are preparing to attack in your absence, it makes them more alive then when would they just sit on their asses hoping that their hero will deal with everything alone.
I would not change this, since curent situation is fine. :P
But you certainly should be able to tell Minthara about secret passage, to prepare another ambush.
It would be even better if you can convince Minthara to send army to gate, where they are suppose to distract and occupy defenders ... and join you and your group within the secret passage ... and from there you either could amsbush defenders from inside, or prepare there another ambush to capture Minthara as they say "with her pants down".
And to make it absolutely brilliant, in finish you should be able to show her restrained and captured to goblins so they surender (or run).
i) Arrange the transport of "oil barrels" , with actually goblins hiding inside the barrels and helping us during the siege, when given the signal.
Since using secret passage will have same result, and since you are no "suply manager" of the groove, but simply traveler, who just happened to arive there ...
I cant quite imagine how exactly you like to justify such action. O_o
k) Destroy/poison the food supplies of either Grove or Goblin Camp, forcing an early attack with weakened enemies or making defenders weaker, possibly even forcing them to move.
There certainly is a way to poison beer in goblin camp ...
Personaly i find it quite sad that only thing you can use it actual poison ... personaly i wanted to use sleep potion with mine pacificst gameplay.

Also im not quite sure what food suply you have in druid groove.
The only thing i can think about is that boiling pot, where that old tiefling is boiling some unspecific brown possibly meaty goo.

But yeah, why not ...
I would also like to add posibility to add there some benefitial potion to on the contrary make attackers / defenders stronger. :P
l) Be able to gather additional allies for attack or defense (like flaming fist/mercenaries) or for example improve the weapons on your troops, by engaging in attack on merchant wagons travelling nearby.
Whole Flaming Fist group is retrurning to Baldur's Gate "as fast as possible" once you help them with their problem ...
Maybe if there would be some absurdly hard persuation ... but even after it sounds a bit odd. :-/
I would stick with them agreeing to escort tieflings, if they manage to join them imediatly, since that way they at least have common goal (reach Baldur's Gate as soon as possible).
Not sure what wagons you are talking about ...
I can imagine trying to convince those so called paladins of Tyr to help defend tieflings ... not quite sure how that would work, concidering they are just hunting one of them.

6) Fight when we first arrive at druid grove
- possibility of joining the goblins, who while get repelled during the attack, lead us back to goblin camp and we get treated there well, instead of the usual mocking.
+1
This is certainly needed ...
Since right now, even if you manage to kill all defenders (Zevlor and Wyll included), they welcome you as their saviour. :-/
But lets be honest, it would require a lot of work ...
On the other hand, it could also quite improve game experience for Drows if there will be some story even for those who are not allowed to enter groove.
Well ... i presume you should be able to stay hidden during whole fight and not parcipiate in it at all ... yet since i allready tryed to kill defenders and i was still welcomed inside, i presume you would still be welcomed inside as "precious helper in such hard situation" right now. -_-
So definietly +1
how badly goblins treat us unless we are drows.
That is something that should be kept as it is ... since that have its reason and its a good one. :P
we should be given better explaination why we join the goblins, like for possiblity of controlling tadpole or learning how it works so that we can remove it later.
As i allready said countless times in countless topics ...
Those things are present, they just arent specificly "told" ... wich i personaly find even better, since those are options for perceptive characters (or maybe players).
The only way i could find this to be implemented so it fits to curent situation that i honestly love ... its simple introspective comentary from narator ...
Something like when you talk to last of Goblin leaders, you sucess percetion check and narator tells you something like:
"Sudently you realize that she have the same tadpole as you do, they all do. That means that they must know some way to avoid transformation ... is their Goddess powers protecting them prehaps?"It still seem stupid, but much better that way, than if some character tells you the same. -_-