bear countryMost of your post is right on (and congruent with what I've said already). A couple of quibbles:
Not so simple at all. Predatory black bear attacks are FAR more common than polar bears - partly because of bear populations, and also bear proximity to human populations (as I touched on earlier). That said, yes, when I'm in polar bear country, I worry more about attacks than I do with black bears. All other things being equal, I'd agree with you - but those other things are not equal at all.
Absolutely. I think that's probably the least understood of all the 'rules' about travel in bear country. "Reading the bear", as you said, is what it's all about.
Not really - black bears are MUCH less of a threat with cubs than grizzlies are. There are actually surprisingly few (documented) incidents of black bear sows w/ cubs being aggressive. They're FAR more likely to haul a$$. A grizzly sow, w/ cubs, generally less so.
Even on a food source/cache, grizzlies are (usually) FAR more aggressive than black bears, and, fwiw, polar bears.
Depends on so many things; for example, in Denali NP, the distance from a griz is about 400 yards; @Katmai, it's 50 yards. And @ Lake Clark NP, there is no stipulated or recommended distance.