Siding with Kahga initially or uncovering the shadow druid plot and joining them definitely seem like either unfinished (waiting for full release) or cut content.
Or ... it was never intended as a questline and you just connected wrong dots ... ben there.

Last I checked if you insist on helping Kahga, you fight and kill Zevlor and it just kinda....ends. I know that there is a resolution to this quest where Kahga initiates the rite of thorns and the grove is blocked off, but I'm not really sure how you are supposed to 'get' that conclusion
Exactly as you just described ...
You just need to report it then, and long rest ...
Not gonna lie, I found the whole Tiefling/Druid conflict very unsatisfying to play through as an evil Player Character-there just doesn't seem to be any real quest infrastructure supporting that option right now, and it doesn't help that although you are presented with the option of helping Kahga, the game clearly doesn't want you to actually follow up on it, since it's essentially a dead end and basically sabotages your efforts if you go through with it since you lose access to the other grove resources/merchants/quests/etc (and even Kahga herself basically says she won't reward you for it).
Its called consequences ... and this one not just makes perfect sense, but you are warned about them by every-single NPC ... yet you decided to do it anyway ... now you have what you wanted.
You dont like it? Well, thats understandable, but not a fault of the game.

you don't even get the option to just leave and let the shadow druids sort things out
Technically you can run out ... not sure what effect it would have tho, i never tryed it before.

Cool
I'm guessing it's safe to say you've explored every possible dialogue choice in EA?
Nah, far from it.

I have replayed it a lot tho.
