I've often wondered how booking at T-355 and then adding a return (10+) days later squares with the ticket validity T&Cs but obviously many, many people have done it without an issue. I included myself in that, having flown outbound and inbound 2-4-1 flights 11+ months apart, probably 23 months after booking the outbound.

I hope you get it resolved. One possible solution if not could be (if you have the Avios) to book the return separately so it would be on a separate booking and then phone BA to get half of the Avios refunded. I've not done this myself but it's often suggested as an alternative to having to ring at midnight to secure the returns on popular routes.