If your fiance buys in his own name (i.e. sole name on house deeds and mortgage) then you have zero rights to the house. He would also have to make all the mortgage repayments. As far as I know, if he buys on his own, then he still is an FTB for stamp duty purposes. Don't know what happens when you get married but it may be just that the house can then be transferred into both your names as part of that (no stamp duty payable on transfers between spouses).
I can't imagine that each member of a married couple can nominate different houses as PPRs, but I've really no idea on that one. If you are living together, then of course, you have the same PPR.
If I was you, I'd ask a financial advisor. Often times, when you hear the answer you want, it's hard to shake it, even if the person giving you the answer may not have sufficient knowledge or information to back it up. At the end of the day, if you got it wrong, you could be liable for the full stamp duty (and potentially interest and penalties if the Revenue took a dim view of it), so unless the "someone" that told you that it was all fine and no stamp duty was payable was an independent financial advisor, I'd dig deeper into the whole question and get a definitive answer from a qualified person.
Sprite