I don't know what the timeline is for returns after receipt of a prepaid return label. You seem to be doing all that can be done to resolve this with customer support.

What I don't understand is why you didn't have the item already packed and ready to go? This may have prevented the  problem, or at the very least put you in a stronger position if the deadline to dispatch the return is unachievable.

Anyway, good luck and I hope you can resolve matters with eBay.