I think Ultra is plausibly scum with Fang. Fang has been looking to tie people to Ultra, not Flower, and he has no reason to lynch Flower over Ultra here. Ultra also has been pretty tepid around Fang - he offered a defence for him but seemed to be caught a little in the middle of the whole thing without any real direction.
I'd rather lynch Fang between the two. I don't think Fang's flip exonerates or condemns Ultra either way, but to me this is cut and dry. If he is doing this as town, then he needs to go anyway, and before I'm accused of this being an out I'm giving myself - you're free to do whatever you want with my slot if he flips town. I only give the leeway because there is the possibility that he is just trolling the game, but I don't think it's significantly likely.