But haven't the account audits been for not flying on the final leg of a journey, which would be prohibited by ticket conditions?

Once I have bought a return ticket, with the return leg e.g. two weeks after the outward journey, the ticket does not have any conditions on what I can or can't do during those two weeks, including booking another return journey in the reverse direction.

While dropping the last leg, however silly the rules are, would be against ticket conditions, if BA ever decided to "audit" me for a nested journey, I would be furious, and they would just push me straight into the arms of EasyJet.