Re: WHAT. Noctis will be slightly distracted by his shirtless Shield BUT he'll be very devoted to best man duties and helping out wherever he's needed! He can warp around to make sure stuff gets where it needs to go fast. He'll give a stammering toast at some point, about how he didn't know Nyx was his brother until he got here, but he'd always looked up to him for how cool and honorable and well-liked he was and how glad he is that Nyx got a chance to find such a good life here. He hopes it's obnoxiously long. Oh, and he'll dance. A little badly but the way people who are having a very good time dance. At some point will have one too many drinks and tell someone "do you SEE this" and gesture dumbly at Gladio's everything, but refuse to elaborate.
Jacob loves a party but he's pretty fond of Nyx so he'll be good. Well fun good, not dull sitting in a corner good. He can't dance to modern music still but he will pull Serefin out for a lazy waltz type thing at some point. And he'll offer to serve drinks if anybody working needs a break? No speech from this one but he will shout "Congrats you handsome knobs" during a quiet lull between toasts.