Thank you for the tips!
Turns out, it’s not the trim that’s inaccurate, but actually the projection, and it didn’t matter that I used your construction options:
Granted, that’s an enourmous zoom, but still… I was able to spot it at normal zoom without even using evaluation.
Absolutely no idea how to make it go away.
Regarding the arc… when you create a three point arc, you get a fourth as well… that fourth point is the center of the radius, and that’s the one I want to lock down, so when I drag the three other keypoints, the “origin” stays put.