Hmm, I know it sound strange but I would almost look into the possibility of spyware // trojans // third party remote monitoring tools.
I know one thing my friends would complain about when I am helping them fix
issues with remote assistance is I'd often need to move my mouse off to the far right corner to access my desktop and/or their mouse cursor would be move to the far right when I started using the remote assistance window again.
I'd almost recommend installing the 'HID Non-user interface filter' for your mouse but that fix
from ms is designed for optical mouses picking up really small movement in the mouse lazer when there are no movements in the mouse itself.
Have you tried switching mouses at all also? Like just using a generic trackball or lazer mouse? Maybe borrowing one from a friend?