I received some useful comments from my bee group about swarms: the first swarm to emerge from a colony is the prime; subsequent swarms are casts. A cast swarm will have a virgin Queen so it will take longer to build up because the Queen has to mate.
My intervention of putting brood into Hive D - which appeared to have no downsides - was probably at least premature. It was pointed out that brood must be cared for. They need warmth and food. This takes bees away from foraging and other duties. This can be difficult for a small cast which is slowly building up. The introduced brood can partially die by chilling or starving.
I think that this cast was strong enough to cope with the shallow frame of brood which I introduced. Much of that brood was already capped, which should give them a quick boost. Be that as it may, there's a good chance that my move was unnecessary.