Consult a lawyer. In the interim, read the entire agreement regarding the guaranteed lease back. For example, are there "weasel clauses" that allow him to rescind the contract under certain conditions?
Also, it sounds as if there may be two written agreements: "a written assurance on his company letter head paper that he will lease it back for 2 years" and "a one year guaranteed lease back." A lawyer can tell you which would be recognized in court: the initial 2-year written assurance which may have been the factor that caused you to purchase, or the subsequent 1-year lease which, if you signed, might take precedence over the earlier assurance.
In your third paragraph, you refer to taking "the builder" to court. Again, I'm not a lawyer, so check with a lawyer. But your contract is with whoever signed the lease back guarantee. If the broker guaranteed the leaseback, then your recourse is with the broker. It'd be up to the broker (on his own) to deal with the builder. Whether the broker would be successful really wouldn't be germain (except that, of course, the broker might need to successfully sue the builder in order to pay you the money you may be owed).
A couple of years ago, I heard all these pitches for pre-construction properties in Florida and Las Vegas, with the guaranteed leases. I wondered what the status of those were today. I guess your experience is an indication of what's happened.
As for good lawyers, I don't know Florida so unfortunately I can't help. But contact your county bar association and ask for referrals. Make sure it's a real estate lawyer. And request one who represents buyers, not builders. Often, the bar association will provide 3 referrals. That'll provide a good start.
Hope that helps.