While I agree with almost all of your post, I always get the FA before I start moving stuff in the bins. The FA will always help you move the stuff and, usually, ask for the owner and if it's okay to move the stuff. Nobody will say no to the FA, and you avoid the jerk passenger going off on you.
When I've put my stuff in the OH it, if it's moved, there is room for someone else's stuff, I have no problem having it moved, but I do have a problem with someone touching my stuff and/or moving it without asking.
BTW, why do people put jackets in the OH if they're in first? Heck, even if I don't get an UG, I ask to put my jacket in the closet.