Sorry to hear you're dealing with this - it's a frustrating situation. From what I understand of how Florida PACE works, the lender is probably going to be tough to go after directly. Their whole verification process is basically a recorded "closing call" plus a Certificate of Completion signed by the property owner - there's no physical inspection of the work. So the fraud, if that's what happened, occurred at the point where the previous owner signed off on work that wasn't done, not on the lender's end. That's why they're not budging, from their perspective, they followed their process.
Your stronger case is against the previous owner and the contractor, not the lender. I'd request the recording of that closing call and the signed Certificate of Completion - if the prior owner certified that a solar thermal pool heater exists when it clearly doesn't, that's pretty direct evidence. From there you're looking at possible claims against the seller (fraud/misrepresentation, failure to disclose), a complaint against the contractor with Florida's DBPR, and a title insurance claim if you have one, since an improperly certified lien like this might be covered.
That said, before spending money on any of this, I'd seriously think through whether the numbers make sense. Litigation isn't cheap, and if you're worried about foreclosure risk down the line, you need to ask whether the previous owner or contractor actually have assets worth going after. Winning a judgment against someone who's judgment-proof (no money, no property, nothing to collect) just means you're out legal fees with nothing to show for it. It might be worth a consult just to get a read on collectability before committing to anything.
Disclaimer: I'm not a lawyer and this isn't legal advice, just general information. Given the money and stakes involved here, please talk to a licensed attorney in Florida before deciding how to proceed.