Yes, you get there either with quest to kill goblins, with promissed rewards wich you were warn about it "will not be much".
Or, you simply get there without the quest, since you dont need to care about Tieflings, nor Druids ... but even then you do allready know that they present potential threat for groove.
True ... there is not much in goblin camp ...
Unless you count temple inside, that will pretty much double it.
Still it may not seem like much, but since goblins just infest abandonned places, than build anything own ... it seem understandable to me.
On the other hand, in groove isnt so much more ... i mean there probably is more things, but not like double, or triple. :-/
Im not sure what do you mean with that vacuum ... like they all are selfish? Ofc they are, they are goblins.
Well ... if main story is geting rid of tadpole, there really are just Minthara, and Gut ... aggain, simmilar to groove, where is only Halsin(potentialy) and Nettie.
IMHO Minthara is more like "go kill the people in the grove, because that is the only possible reason you are here" ... all she know about you is the fact that you are true soul, and one can understand that someone like her would presume that you two will not talk, if you were there to kill her ...
Also i was there so far talking to her only with Sazza ... Sazza is talking about posibility of attack, and you have no objection, so that is another reason to presume you are there to join the attack.
Why create leaders ... who do you mean?
Goblin leaders that Halsin want you to kill? Probably bcs quest "go and kill five completely random goblin" will be quite stupid.
Or do you mean Zevlor, Halsin, and Kagha? ... I gues for same reason as Minthara, Dror Ragzlin, and Gut.
I gues if you dont specificly do what are you specificly told not to do ... at last one of leaders from each side should be alive, more like 2. And i gues every one of them will have some story in future, wich will depend on wich one of them survived.
And second question ... you ask why would any character join Minthara.
I hope reasons for mine chaotic evil Drow will surfice:
1) Its a Drow, i know that this race isnt known for some racial loayality, but still my character is closer to her, than some surfacedwellers.
2) Why not? My character dont care about litteraly anyone in that groove, and raid with goblin army sounds like fun.
3) Multiple persons from groove did repeatly insult my character for being a Drow ... it may not seem like much, but those simple reveges warms her cold heart.
4) Minthara is servant of the Absolute, wich as it seems control mine tadpole ... so my chances with someone who allready have control over my parasite seem biger, than with someone who may study some parasite for some limited time.
5) There is Cult that worships Absolute, wich potentialy promises power.
6) Absolute promises better life for Goblins, Hopgoblins, Ogres, Duegars, Drow, etc. ... ofc. my character probably see "better life" differently than that goblin, that tells her about it, but that is nothing she will care about.
7) Every treasure, every valuable, and every artefact i found in groove will be mine.
8) Bcs screw druids, that is why. :P
9) And finaly and maybe most important ... its evil
Hope this form of answer will be more "readable" for you, dont get used to it ... i dont do that often, i writen it all as usualy, and then just deleted your quotes, i simply cant write anyhow else, since then i loose myself in it.