So, procrastinated and now have 8 cards to use up credits. Split between 3 players.
Best way to do it is:
Buy $50 GC
Find a ~$250 flight. Apply $50 GC and $200 from card 1
Find a $450 flight. Change my flight to this flight which will apply $250 credit. Then use card 2 for last $200.
Find $650 flight. Change flight as above.
Do this for each player.
Cancel after 48 hours or book very far ahead in time to allow more time to cancel or just change flight in the future.
Looks like I should use main cabin based on above?