+1 to squashing commits for PRs prior to merging them. The "fix blah" commits make it difficult to figure the full contents of the PR and arguably nobody cares about the sausage factory aspect of it. If there's an actual fix to something broken in a PR then it's a separate PR anyway.
I'm neutral on the no-ff merge commit for PRs. It's really convenient to click to merge from the GitHub web UI and it gives decent place to put the PR reference # (which I think GitHub autopopulates for you). Otherwise the original PR author has to create the PR, then update it again to add the "PR # blah" line in their own commit. Not a ton of work but anything circular like that is annoying.