how raiding the druids (aka the actual evil path split) will help you at all
There's more of then, and they are more likely to win. So just siding with the winning side. Not like the thieflings are offering any help to you. They are weaker, and less useful to you personally.
Druids sit it out, they lock themselves in their grove. They were offering help, but they also want the thieflings out anyways. Really, helping the thieflings isn't even neutral, its straight up a "good" coded decision. Its altruistic, you don't benefit from it.
The neutral decision would've been to wait for the druids to finish their ritual, wait for the thieflings to get on the road (and probably get massacred), and then negotiate passage into the Underdark or seek help from druids or whatever. But this isn't an option here, as far as I know.
No, that is meta knowledge and not pointed in the game dialog or quests on top
You feel forbidden to assume, or to think any thought not explicitly spelled out for you by the game "content"?
This actually is spelled out by the game, but I think its in Act II, when the guardian straight up says "its in our benefit to join and infiltrate the Absolute cult". Not sure if there's any such line in Act I. But also, its obvious.