I finished the mayor's quest, but cannot return to the temple/leave Thessila.
confirming again: you went through the quest to defeat Merile, right? Or is this only after you defeat the tree and you return to the temple so the mayor thanks you? (He asks you to spend the night at the inn). All possible endings to that only ends in a return to the temple - I've been retesting it and so far it works for me, so am looking at other possible errors on my end!
I fixed it with a new save since I wasn't that far in the game. I didn't get a quest to defeat Merile, the questline ended when I refused to dick down the tree lady (On accident). She died, and then the quest with the mayor's office didn't begin, and the tree remained there. The only mayor quest I had was to check the tree out, which I was able to collect rewards for/complete.