I think it is very very unlikely that you will fail to match. It is not too late to apply to more programs. If there are some that are more competitive that you avoided due to fear, you should just go ahead and pick out 4 or 5 of those and apply, IMHO. Nothing ventured, nothing gained.
I think that for IM, even if you have some sort of defect in your transcript, if you go to 8 or 10 interviews the odds of not matching anywhere would be extremely low. In your situation, you might want to hedge your bets and go to 12 interviews or something. Perhaps you can fix is so that you have back to back interview days in the same city or cities, to save money.
It is definitely all right to turn down or cancel interviews, and I with the number of applications they get in IM, I doubt programs would even remember the next year. Also, you would have/would have had a good reason for turning down some of the interviews...you just could not afford, time and money-wise, to go on all of them. I actually turned down a few cardiology interviews - it is an O.K. thing to do. The way you do it is you drop them a very polite email saying that you were very flattered to be offered the interview, but you ended up with just too many interview invitations and you could not accept them all, and you wanted to let them know ASAP. The only thing you don't want to do, if possible, is to wait until a few days before and then cancel - that's rude if you can avoid it.
You could do what was suggested above and just hang on to interview invited until Nov 1st and THEN cancel the ones you don't want. However, if you already have 30 invites, then it's time to start culling the list now, b/c you can't go to that many interviews.