@Beebe i read an interesting paper by Ralph Buchler recently, mainly about queen rearing but also uniting 2 colonies when one queen is to be removed. So slightly different situation to yours but nonetheless one queenless and one queen right to be united.
His advice was to cage the recipients queen first, for a week. Didn’t say why but I presume as her pheromone is reduced maybe gives the message the queen is starting to fail, so more receptive to uniting. Then unite after removing her in the cage.
He goes on in the paper to also recommend putting the nuc / Q+ colony with the mated queen you want to introduce / unite, above a double screen with the recipient colony below first before uniting. He states this has even more success at 95%-100%. Again doesn’t say why but I presume as there is mixing of colony odours, but no trophallaxis, the introduction is really slowed down. Why don’t authors of insightful work give a behavioural explanation behind their research!?
I have a colony that has a new queen but 6 weeks on from emergence she isn’t laying yet though I’ve seen her and she’s mated. They’re a feisty bunch that follow, hence the attempt at requeening so my next tack if this queen doesn’t start laying next week is to follow Ralph Buchler advice.
Queen introduction seems to be one of the trickiest areas of beekeeping, when a colony has either feistier bees or more than the usual proportion of older bees