Update... so, I finally just gave up. I'm going to just travel as originally booked.

I'm curious if there's a path here for compensation from BA, as they've basically forced a fully flex ticket to be completely inflexible - in effect removing a key feature of the ticket I purchased. Especially given it was BA's fault (cancelling the outbound) which led to this. I might mull this over.

TLDR; Don't use travel agents if you need any kind of booking flexibility. Definitely scrapping the Amex Plat now.