assuming they’ve already swarmed.
Always worth a check, especially if eggs are seen. Several times this year I've found very strong colonies with sealed QCs and eggs, but went through and found queens (some clipped, some not). Rough guess is that variable weather led to delay, but then I found three last week when it was ideal swarming weather.
adding foundation is not adding laying space
My recollection is that standard advice referred to the addition of supers of foundation to give an expanding colony somewhere to put bees & nectar, other than in the BB. Awkward if you're starting off and have no comb at all, but give foundation anyway.
If a colony needs laying space then take out stores and add a frame or two of foundation into the middle of the nest, alternating the foundation. Check after a week and repeat if necessary. Can be done on single or multiple BBs.
Sometimes the flow outpaces the queen and bees may draw and fill the BB foundation with honey. Move it to the edge of the box and give another foundation in the middle. Essence of this is to give them work to do and avoid the point when colony maturity is reached.
do a demaree (but I’ve never done one before so I don’t know!)
Nor me.
find an emerging virgin then you should release all the other virgins from their cells rather than destroy them
When you find a colony with emerging, sealed or open QCS, or with one or more empty, emerged QCs, the bees have the opportunity to swarm as each virgin emerges. When you remove the roof and open up, the bees preventing queens from emerging will be dispersed and more virgins will start coming out.
By releasing all the virgins at the same time the colony will select (or the virgins fight) to reduce to one, and
no swarm will issue provided you remove at the same time all other QCs. Be warned: if you miss the tiddliest nub of a hidden QC they will very likely swarm on the chosen released virgin, knowing that the tiddler will take over.
You don't have to let them all into the hive: cage a few and make up three-frame nucs (one of sealed brood, one of stores, one of something else, three shakes of house bees). Good SQCs can be used similarly.
If you do destroy QCs I believe it's better to remove them: bees have been known to repair damaged QCs, and all your work will be undone.