Example 1: you come to the gate and observe the goblin attack. What would evil characters do? Will they help? No, they don't care. So, you just stay there and enjoy the view how the goblins and defenders fight without any interference, i.e. you skip turns (if you're behind the hill then goblins don't attack you, or you can enter a stealth mode). And, of course, the plot immediately falls apart. The game always assumes that you heroically help the defenders and all dialogues include options to emphasize your role in protecting the grove. Everybody praise you for the deed you didn't do.
1) As far as i know the encounter is set like that so you are allways in sight for at least one goblin so even tho your *evil* character would not go help they will probably defend themselves.
1. No, it is not. Are you trying to tell me what was my in-game experience? Stop it.
2) You seem to more than willingly ignore the fact that not everyone in your party is evil, and the others would go help ... unless you play with full custom party of totally evil band, or coming here allone, wich is both creating ilogical situations on purpose ... every story can be broken if you know what to do.
2. All the companions in your group can be evil (Lae'zel and Astarion are 100% evil) and you easily can come there alone, being evil. It is not illogical situation, there's even a route you can take where you meet no one.
3) Once you harm even a single enemy like it or not you just helped.

4) Yes they praise for something you didnt do ... what kind of evil character would mind it?

3. As I said, it's very easy to just observe combat without participating.
4. I wouldn't mind it if it makes sense. And it does not.
Example 2: also in the grove, when the druids deny you entry, what would evil characters do? Who are they to tell me what to do, right? You can just ignore them and they attack you for trespassing... And the plot again falls apart because the game assumes that it is you, who attack them on behalf of Zevlor even if you didn't talk to him at all, and they start attacking tieflings out of the blue. All the dialogues later no longer make sense because they loose continuity. At the same time if you just jump into the grove from the side, practically doing the same trespassing, druids just tell you to behave and won't attack... because logic wasn't there.
I never managed to get around them. O_o
Unless once again metagaming on purpose.

Just jump down where the elevator is, no metagaming necessary.
But lets say you did ... since you are talking about logic ... how are druids suppose to know that you talked (or didnt) to Zevlor?
I mean you are not one of them ... therefore its quite logical that they concider you to be more likely tied to people who are disrespecting theyr rules for last few monts than to expect you to be just some random dude who came here "just to cause problems".

Exactly. They don't know. But in the game they assume that some random travelers that just arrived already conspired to kill them with Zevlor, when all you do is to tell them that you need a healer and that you go where you want. A clear case of conspiracy, right? And if you jump to the circle from the back and sneak around then it's totally fine... Because logic.