My understanding is that you have to pass CK in order to participate in the match now and if you don't have passing scores by the deadline they withdraw you. Nobody other than the Dean's office knows that I failed so I'm still going to interviews. I have one derm and a TY left.
My Dean did say I could extend my schooling so I can be a US senior rather than US grad in the next match but I don't know how much that matters to programs.
This is incorrect. Only IMG's must pass Step 2, because they need it for ECFMG certification. There is no such requirement for US MD's and DO's. Some individual programs may decide that they require Step 2, and if you don't have it they will remove you from the list. But that's it. So you can still ride out the match, and see what happens. And if you do match, they can't dump you once they find out. The match is binding on both parties. They may not be happy about it.
So, Plan A: Continue with match, hope for the best. If any program asks for your S2 score, either don't respond / play dumb, or email them your result. If you re-release it in ERAS, everyone sees it.
What to do if you don't match derm this year is more complicated. I pointed this out above, but it's actually a bit more nuanced.
One thing is really important to hear: If you don't get a derm spot this year, you're not getting one. Continuing to work towards a derm spot will likely end in misery for you. You already have a research year -- another will not help. You'll have a failed CK score. You only got 3 interviews this year, you'll get less next year. You really need another career plan if this doesn't work out.
Options:
1. Put your TY/Prelims on your main rank list, starting at #4. If you don't match derm, this gives you the chance to match into just a prelim. This then allows you to re-apply for derm positions next year (bad idea), or transition into a PGY-2 year in IM (if that's what you see yourself doing). You would also be able to SOAP into any open A positions (derm, if any. Or Rads, if interested. Or others).
2. Don't put your TY/Prelims on your main rank list, and SOAP for Categorical spots.
3. Do none of those (or try, and not SOAP), and either extend training, or do something for a gap year. In either case, my advice would be to re-direct your career in a different direction. Else, if you apply to derm and a backup field, there's a reasonable chance that the backup field programs will look at your app and dump it, recognizing that you really want derm.
Regarding extending medical school to apply as a US senior vs graduating and filling a gap year, there's no correct answer. Conventional wisdom here on SDN is that it's better to be a US Senior, because if you look at NRMP stats, they match at 95+% and previous US grads match at a much lower percentage. But that's a very skewed comparison, because the previous US grads are presumably concentrated with people who had application problems like yourself. Staying a US senior doesn't give you a leg up in the match. What it would do is allow you to do a bunch of new rotations in new fields to find something else. It's def the best idea -- as long as your school isn't going to charge you another $50K for it. In that case, it's a tough call.