Id get in there with my child fingers and scratch the wad of gunk off. Usually id end up wedging it in the housing but thats my dad's problem.

Yeah, you kinda had to at a certain point, the gunk would inhibit the ball from rolling freely.

There was always a kind of lint that would develop on the wheel inside. I've got no idea what it was made out of.

I remember cleaning the ball once or twice, yes, when it got too bad to ignore.

Mostly the gunk accumulated on the rollers, though (and under the mouse, like in optical ones), and that was easier and faster to just scratch off with your nail... except for the little third wheel (seen on the bottom left picture) that kept the ball centered, of course, because that one wasn't only smaller but on a spring, making it almost impossible to scratch the gunk off of.

There was also the occasional hair tangled up on a roller, of course, which was almost impossible to remove. Those you just pushed aside onto the roller's axle and hoped the mouse would die of some electrical failure before the poor thing got too full of hair to roll.

Yes.