Thanks, appreciate the responses. Wasn't being facetious or anything asking if it was reasonable, just have little experience with both OTA-linked stuff as well as credits for partially-flown itineraries. I think the best course of action is to try to use the credit via Amex Travel to book a valid round trip for close to the full amount (eg DEN-LHR), then try to see what I can do with modifying that new reservation.