Taking advantage of whatever formal protections afforded you is a smart idea, but at the end of the day you cannot protect any idea in the abstract.
The pre-registration requires some history of infringement, or at least that was my understanding. This implies that there may be a question concerning your rights anytime that is the case it is best to consult a lawyer.
I suggest that you consult with a lawyer in private and discuss your objectives in more detail. You can start by calling around to several for a free phone consultation, get some insights then pick the best fit to work with.
DISCLAIMER: this is not intended to be specific legal advice and should not be relied upon as such. No attorney-client relationship is formed with the law firm of Natoli-Lapin, LLC on the basis of this posting.
Your questions here and in your previous post suggest that it's time to speak to an IP attorney "off line".
You are not likely to have or get copyright protection for a "theme". You get copyright protection for the expression of the theme in a work of original authorship fixed in any tangible medium of expression. So, I think this will all be for naught on your part, unless you are misdescribing what you are trying to protect. Near the end, you switch from "theme" to "advertising and marketing photos" - which is it? I have no interest in trying to figure out for free here. You need to hire an attorney at a reasonable fee to review this. Frankly, it is unrealistic to hope to get 100% assurance here with such ambiguous, subjectively stated, conclusory and minimal facts, and even nuttier to hope to get it for free.
Most of what you describe is irrelevant. Background noise to us lawyers, such as: you hired a model, she mentioned it to some group, you hosted a Benefit, whether they came up with the "idea", when whoever registered in FL (and whether legally or not), whether you "know" its a 100% lie [in Court it's the perception of the truth as gleaned by the Judge or Jury from a preponderance of the admissible evidence of record that matters not the actual truth as the actual truth is seldom known for certain], etc.
No, of course the pre-registration will not 100% protect you. The Court may conclude you are the liar and invalidate it. Again, depends on who has the better evidence if this is litigated. My guess is the group will have better witnesses, but how the heck would any of us attorneys know without reviewing the matter?
If you want 100% certainty, you need to buy insurance or get a direct connection to GOD, to whom all things are possible.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.