Thanks for the replies!
Ok - with my crazy breadboard setup, I've found that disconnecting pin 14 and connecting it to pin 10 on the floppy side makes the drive work.
Just formatted an HD floppy successfully. Yaaay!
I think these slim drives might make some of the signals 'multi-purpose'. Still, it doesn't really make sense because I'm apparently connecting DRVSA to MOTEA... which doesn't exactly fit with the idea of swapping DRVSB/A.
I'm off to make a ribbon cable!
And for posterity, here's the craziness, which, now it works, I'm quite proud of
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