I used to be that guy, all self-righteous that someone could take a handicapped spot without visible disability, until someone called me on it. So embarrassing, but completely deserved and a valuable lesson

I don’t know if we can generalize from my personal experience but at the time, I leaned more conservative. My outrage was that someone broke the rules, was getting away with something unless someone called them on it. Of course now I understand the result is more important than the rules, that people can have all sorts of physical limitations, visible and invisible, that most people do the right thing most of the time, and “who am I to judge?”

My father-in-law has disabled vet plates, which helps avoid confusion, but to look at him you wouldn't assume he's disabled.

This is actually the rule if you are a transit operator. I would however ask someone to not take up 3 seats.

Yup. Worked in workers comp. There's tons of fakers that are too good to spot, and then there's goofy motherfuckers who you'd swear were faking but actually were not.