I confess to not checking T&C's for GC's, nor the FAQ"s of late. Although, I doubt this is a new policy.
See more below.
Almost missed this since you didn't properly tag me as @Steelers36. 😉
Keep reading....
You may be on to something. Keep reading to below.
Seems like you got conflicting responses. First, NO to EZ-Air, but then YES to Flexible fares. (Aside: Does anyone actually booked restricted with EZ-Air? What would be the point of that as cuts you off from price drops?)
Your report seems to match my situation.
We have four voyages in October. First three are B3B on REGAL. All three REGAL's have FCD deposits. The first voyage also has our FCC's from Covid cancellations (we had never paid those in full). There was not enough FCC to fully pay for the cruise portion or the airfare. This first voyage also has our outbound EZ-Air flight (Flexible naturally). Gift cards paid off the balance owing recently and there was no question nor issue about using them to pay the balance.
Other gift cards were applied to Regal 2&3, and small balances were charged to Visa. No air on those cruises.
Our final voyage is on ISLAND, and it includes our return flight (also Flexible EZ-Air). I have also applied final $2,000 in GC's as a partial payment on the account balance. Deposit was FCD's. The GC amount does not cover the cruise fare, but would cover the airfare. This booking is not yet paid in full, but I might request early ticketing before FP Date. If I do, I can report back since the airfare has to be fully paid in order to ticket early and the only way it could be considered paid today is if the GC amount was accepted as payment.