eCredits supports EUR, MKD, CHF, PLN, GBP, SEK, HUF, NOK, DKK, CZK, RON.

In Profile -> Edit Profile, both in the eWallet App and Portal. 

Yes, given it is a profile change, all your platforms and devices will have the new preferred fiat. 

No, for now, the only payment methods for subscriptions are eCredits, Euro, and Voucher. 

No, Instant Trade only exchanges ECS to Euro.