I'm not an expert.
Obviously not, but at least you have a realistic assessment here. It's just that XSS doesn't work that way.
-There are some small % of newb/dumb bitcointalk forum members who probably use a similar password on bitcointalk and on their favorite web wallet. It seems possible that Theymos could somehow get access to people's bitcointalk passwords if he wanted, giving him or his friends a huge password cracking advantage over a random person.
That is true, dumb people will get raped, probably not by theymos though. Or at least not directly.
-Theymos seems to have been a defender/supporter of TradeFortress. It's possible that they are friends, but I'm not sure. The circumstances around 1 million dollars disappearing from TradeFortress's inputs.io business suggest that TradeFortress may be very shady.
TF looks like he's been sloppy, probably not malicious though. But that's just my opinion.
-There are a lot of people on reddit who pop up with stories about how funds were stolen from their hot wallets because they didn't have 2-factor authentication, and they describe having pretty complex passwords. It's somewhat of a mystery how their passwords are being cracked.
I think there are far more plausible explanations the one you're suggesting.
-The fact that Theymos allegedly raised like $600k for this forum, yet the forum is not that good technically, is sort of weird and makes me wonder how much of that was actually spent on the forum.
You seem to assume that transforming money into working software is easy and frictionless. It just isn't, managing an IT project is a full-time job.
I'm not saying we should suspect Theymos of any nefarious activities, but the above is just why I care more about Theymos's trust level than I care about a random user's.
You shouldn't need to trust him if you use a different password for all sites you care about and if you assume no PM privacy.
Bottom line, if your security depends on trusting theymos I suggest you review your security instead of looking for reasons to trust him.