Seeing how the artist behaved in your screenshots and their replies on their FA upload, I would personally just fully refund them (if at all possible) and resell the slot, even using the already made background. And get FA take down the commissioner posted picture, of course.
While I guess they might've over-hyped themselves during the wait time (which they're partially responsible for themselves!), their behavior is being absolutely horrendous. I understand wanting what you paid for, especially when you pay a lot, but for the life of me can't really understand what the commissioner actually wanted the end piece to look like, and I don't think the commissioner does either. You asked for speciifcs and got nothing. Their responses and comments read extremely passive-aggressive, and after reading the submission comment on FA I'm not inclined to give them the benefit of the doubt. Add in charge-back they initiated after being told they would be refunded anyway, and this person becomes someone I absolutely would never do business with.
I hope you can get a reasonable resolution to this all.
