Not sure I see the need to give coins for credit purchase, are the credits not enough? Or is this some way to incentive gifting credits? To get rid fo the 2% stat boosts?
I can understand the point about coins being seen as achievements, to me that adds to their monetary value - you either spent a decent amount unlocking and getting stuff in the shop, whether by supporting the game or grinding free credits or they were a reward for some activity - maybe questing or helping out. I earn my money but I could phrase it as a reward for my acheivements! Maybe I will do so in my CV somehow....
I find it hard to see a clear-cut distinction between barter and pay personally, I suppose they are different but not much functionally? If you are happy to be paid in sand, more power to you.
If you think a GC is too valuable that is fine, give them a SC or three. They do still exist, even if they are a pain to transfer in bulk.
I'm not a fan of the 'If it ain't broke don't fix it' but this has been unchanged for over a decade, was decided by the players (still is) and I've yet to see any evidence it is not 'working'.