Yes, the SWIM chip uses a 16MHz clock which, as you note, isn't sent to the IWM socket on the Plus. I suspect that feeding a SWIM a half-rate clock would make it not work at all, but that's only a guess. But shipping a 16MHz clock signal over to the chip is not hard (a bit ugly, perhaps), so that isn't a showstopper if you're determined to give it a go. Several accelerators (like the Brainstorm) rely on tapping into the 16MHz clock available on the motherboard, so it can be done.

As to a fabled 256K Mac, perhaps it would have used four banks of 64k bytes each.