> (BTW, is it kosher to force push to PRs like this?)

We always squash merge, so you don't need to do it to hide your history. Personally, I do it occasionally when the history is very long and nobody has started reviewing.

Force pushing will reset any reviews, so you'll be forcing reviewers to start again instead of just looking at your updates. Some reviewers (e.g. me) will often defer a full rereview if we don't have a lot of time and someone just force pushed. So in general, avoid it.