I don't believe that you can use free night certifcates for somebody else, but you would have to call Honors, not just change an account number online.

But presumably you are going to be in Seychelles for longer than 3 days. You could easily book the first nights from the Diamond account, and then mention at the hotel that you're staying 3 nights longer under another account. IME hotels that are generous to Diamonds are also generous to Golds. The only major difference in guaranteed benefits is lounge access, so unless you have some other reason to want the Diamond member listed as named guest for the entire stay, you ought to be fine anyhow.