Hello,
Apologies if these are basic questions -- I mostly flight on Oneworld airlines. I've tried to look for answers, but would appreciate if anyone could confirm / add some colour, thank you!

I have two FFCs for non-refundable economy flights that I cancelled. One expires in October and the other in December.

1. Can I combine two FFCs (both in my name) to pay for one flight? -->ChatGPT says yes but not sure if true
   1. When I search a random flight and try to use the FFCs on the payment screen, I get an error message "We're not able to apply this credit here. Follow these steps [...]". The only thing that appears to work is looking up the credits and then searching for a new flight from there -- but that doesn't allow me to use two credits to pay for one flight
2. Can I use them to book flights in a higher cabin than my initial booking, and pay the difference? --> ChatGPT says yes, but when i tried it would only show economy options
3. Can I use them to book flights of different "types" (e.g. inter-US even though my original flights were transantlatic)? --> ChatGPT says yes, but not sure if true
4. Can I extend the expiry date? --> From the above, it seems to be a clear YES, just book another non-refundable flight using the FFC, wait 24hrs, and then cancel it, to get another 1yr extension

Are there any other tricks that I'm missing apart from extending the expiry date (perhaps to combine FFCs into one)?
Thank you very much!