Yes.
Depending on the trim. If the trim is exactly 1500, then it is 1500. But most times it will not be exactly 1500.
I didn’t think about transition from Q-modes. When I wrote the post I had (non-Q) transitions between manual and stabilized modes in mind.
When you transition from Q modes the angle-controller is already active, as you mentioned. I don’t know enough about that to say if those transitions are effected by auto trim. My intuition would say it shouldn’t have an effect from one stabilized mode to another stabilized mode, but I also have no idea how exactly the code handles Q to non-Q transitions, if control loops get reset or something.
Keep in mind 1) I don’t know how much time it takes to learn and 2) that depends on the trim and how much out of trim the starting values are. And after the trim was learned once every starting point will be much closer to the later learned values, so it will have less effect on every transition after the very first one.