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.
If you’ve already unlocked Ultimate on the character then I don’t believe you can do anything except complete the quests that reward skill and attribute points on Normal/Elite.
Only ones I think are Kasparov’s quest for 4 Aether Crystals, slaying Lutra in the Broken Hills and the Hidden Path (which might be a bit lengthy/tedious).
A lot of other skill/attribute points like the ones from Shattered Realm are Ultimate-exclusive thankfully. Saves players that don’t have merits from having to do a ton of side stuff to max their characters out.
You can launch GDStash, load your character, change highest Campaign difficulty to Elite and then you can use the merit to get those skill points and attributes. Back up your saves / don’t use cloud saving if you do.
If you don’t, quests that awards skill points / attributes that you still haven’t done with your character can be checked in Grim Dawn Checklist or there’s a list of all of them here Quests - Official Grim Dawn Wiki