Okay!!!! Got USB optical mouse working on Atari 2600 with SpeedLink and Willingham adapter. Needed USB hub, had to plug Logitech keyboard in other USB port and mouse into PS/2 port. Got keyboard working first by putting in USB port. Mouse button did not work and could not re-assign space bar to mouse button....then realized, since I was using PS/2 adapters, maybe I had to reassign "X", even though spacebar worked on keyboard. This worked!!! I found on the 2600, it works best to use Esc, Insert but not Esc, PgUp, since it is digital. Mouse is very controllable, not crazy slow or fast. More to come, such as 7800 and trying with Innovation SmartJoy adapter.
Wow! Congrats- that is great. I just now am reading through these posts of yours and it's all very intriguing