I know a number of people have failed step 1 and still matched into PM&R. I don't know about comlex since I'm less familiar with it, so I don't know what a 475 means. But if that's a solid score and if your clerkship grades and LORs are all strong as well, you'd probably still be able to get into a good program. If everything is less-than-stellar, then you still have a chance at PM&R--there are definitely less competitive programs out there.
Regardless of how competitive you think you are, I think it's worth applying if PM&R is your first choice--just apply to every program out there if your concerned about competitiveness. And apply to primary care positions in addition if it really looks like an uphill battle.
But regarding your question about looking up programs that have trouble filling, I don't know other than to try and look up old unmatched lists. This year only one program didn't fill, but I don't know which one it was (only applicants who didn't match are privy to that information, and they are not actually supposed to disclose that information).