Hm, you can’t unlock Ultimate by completely the first act of Elite. If you finish the base game on Elite that unlocks Ultimate.
And no, merits won’t work with characters that have already reached a higher difficulty. If you’re already on Ultimate there’s simply no point to using a merit. If you haven’t done the relevant quests on the lower difficulties you’ll have to go back to do them. Merits are meant for characters to skip up to difficulties, not unlock them when they’re already there.