I agree. On most any decent airline, if you remove a part of the seat, that seat would get put out of service and the cabin size drops by one. At that point, I would expect that if the cabin turned out to be oversold then the carrier would download/offload according to standard procedure when oversold. Look for volunteers first... if none, downgrade lowest fare/status pax (all the while offering appropriate EU-mandated compensation). None of this seems to have happened, but when engineering hauled a part of the seat off the aircraft, somewhere it ought to have been noted that that seat was no longer appropriate for a CW passenger, and should have been taken out of inventory, off the manifest, whatever. This would have triggered exactly the type of re-allocation within the cabin that the OP suggested, but it's BA's pretending "half a seat is still okay in CW" that pre-empted everything.