Aright, so here's the resolution.
Within about 5 hours, they'd ruled in the buyer's favor. Then this morning she left me negative feedback.
They took $80 from me and sent it back to her with this note:
"Unfortunately, it looks like the refund you agreed to issue was never sent to your buyer. We've issued it and now need you to reimburse us."
So this morning I called their customer support to appeal and discuss, because the reason for finding in the buyer's favor didn't seem to match up with ANY of the details from the case. I explained to the nice woman that the listing was clear, I had asked her not to return the shoe, that it was an obvious case of buyer's remorse, that she'd lied when she said the item wasn't as described, and that she'd been belligerent in her contact with me.
I explained that I had listed no returns, no exchanges and questioned what that policy was in place for if any buyer could just drop the item in the mail against my request and then demand a refund on false pretenses, and that I was concerned that whoever ruled the first time didn't really understand the details of what had happened.
So she put me back on hold, and 5 minutes later reversed everything.
They're refunding me, they're refunding my eBay fees for that listing, they're reversing the decision so I don't have a strike on my account, and they're going to remove the negative feedback.
So, I suppose I win for now even if it was a pain in the ass.
EDIT: Got off the phone 10 minutes ago and already have the refund, and the negative feedback is already gone.