Update

Third call, got an excellent agent who understood immediately and was able to apply the GUF2 to my Amex 2-4-1 using A-class availability.

Issue: when pulling A-class, their system was showing a large cash element (around £12k) and they could not get it to price as just the taxes and fees, so it has been sent to back office for manual repricing and ticketing.

They provided a new PNR and confirmed the flights are locked in. I can select seats. They said I should receive an email within 24 hours asking me to call back to make payment.

Will update again once ticketed and priced, or if new troubles arise.