For the first one I tried every spells I had but nothing worked (maybe a secret quest ?)
For the statue I don't know, maybe it's just a "statue"
The tree spoke to Sebille if you have her in your team and told her to take revenge and become godwoken
(If I remember correctly)
Owin... just take his sword and that's all, he'll just walk away
The merchants are just trapped xD and yes you have something to do with the red prince story (if you have him)
And the Undertavern I don't know
Maybe it is a part of the decor...