> What's the queen excluder for if the bank hive is queenless?"

Two reasons. One, to keep any rogue queens from getting into the area where the banked queens are (from outside). Two, to confine the queens if one gets out of a cage, -- or if a virgin hatches from a queen cell on the brood. 

Brood should be minimal in any case, it's just there to keep the bees near the queens and maintain some sense of normalcy. In fact, an alternative is to not have any brood at all, and add nurse bees from other colonies on a regular basis.

You could instead simply use a single story queenless hive, if that seems simpler. The queenless, broodless configuration seems to have the fewest surprises.

PLB

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