I aalways give mine 2 deep in spring, most likely over winter with two. I do my first inspections in Feb. and March. As soon as the 2nd deep is well on the way with brood I reverse brood chambers.
I don't know if there is a good strong colony in this area that can make out on one deep without something
going wrong, swarming, moveing up in a medium super, etc."Unless" they are split, Or take a nuc from them, or, give a frame of brood to another hive every 4 or 5 days.
My march swarm had three deeps on by the middle of April. I did not reverse it.
Ofcourse they had drawn comb to work on.
I would move the super with honey on top, move the mediumbrood on bottom,"IF" most of the brood is in it, then the deep brood in 2nd place. If we have any amount of flows from here on out the summer, Once the Queen and brood moves back up into the deep, Put it back on bottom, then another deep on 2nd place. The brood in the medium in 3rd place will be fine, the nurse bees will not leave them.
Make sure you do have the queen back down in bottom when you do this final move.
If the number of bees have declined , do all of the above, (except) putting the other deep on.
Are you sure they swarmed? some time a crowded colony not prone to swarming, The Queen just doesn't have any where to lay eggs. But this shouldn't cause a large decline all of a sudden.
Thats what I have done