Wow, that's a lot of violence you've run into. I have to say that I've been riding TriMet for 20 years and I haven't run into that much really violent behavior, especially aimed at me. (And I'm no one special, just a fat old white broad.) Who knows what to attribute that to -- probably luck, more than anything else. That, and I live in Beaverton.
Did you report any of these incidents? I keep Tri-Met's number on my cell's speed dial, not just for scheduling checks, but to report incidents like that, too. I know you were late in the last incident, but you might've at least reported it without staying to file charges.
Bus drivers should not be ignoring those incidents. I'd be a squeaky effing wheel each time (and, on the very few occasions I've seen people be violent, I have reported them to the driver. I do actually practice what I preach.) I know these poor drivers probably get jaundiced after seeing a lot of it, but that doesn't mean they can just relax and let their innocent passengers get abused. If you're afraid of retaliation, though, then I dunno -- just wait, get the number of the bus and the time, and call it in later. Most have security cameras these days, maybe that could help them do something.
I live in Beaverton, and we've got the whole suburban white-privilege thing going here. There aren't a lot of halfway houses, and people who are off their meds don't come out this way. They probably get arrested or something if they do. I hate elite privilege, but I'll be candid and say I don't miss the crazies, even if that makes me a bad person.
To you and the OP, I'm sorry to hear such scary things are happening. Good luck on avoiding such abuse in the future, to all of us.