Your questions have already been answered (correctly) above, but I'll add my 2 cents.
1. If you do not obtain a prelim, you will lose your Ophthal spot. If you lose your Ophthal spot, you are very unlikely to ever get another one.
2. Ophthal allows for a TY, Prelim IM, or Prelim GS position for the PGY-1.
3. It's not a match violation to not take a spot, and lose your PGY-2.
4. You seem to be on the verge of making a terrible, terrible mistake. There will be prelim IM and GS positions in SOAP. if you don't match to your TY, you should seriously consider applying for and accepting a prelim IM or GS position. If you don't, any career in Ophthal is over and you'll be applying again next year for another field. And everyone is going to ask you what happened this year. And when you answer "Well, I got an ophthal position but then was only willing to take a TY which I didn't get so now I'm applying to your field" it will not end well.
All of this assumes that you've actually matched to an ophthal position. if not, then it's a very different picture. You might decide not to SOAP into anything and instead take a year of research in ophthal and apply again next year -- that would be a very reasonable decision (although in that case you should have a plan B for next year also if you don't match again).
So, bottom line:
1. If you've matched into ophthal and that's what you want to do, get a prelim year in SOAP if you don't match to your TY. Failing to do so will be the end of your career in ophthal -- you will not just get a "do over" next year.
2. If you no longer want to do ophthal and you've matched, you can resign. Sure, it's a match violation in the SF match but who cares?
3. If you no longer want to do ophthal, then you should consider SOAPing into the field you want (if you don't match to your TY).
