You can't take the car back under false pretences and then simply 'reverse the deal'. Name on the V5 is irrelevant. If your customer has bought and paid for the car then it belongs to them (and presumably there is a paper trail to prove it). If I've understood your post correctly then what you're proposing is basically theft and can land you in deep trouble. You would not stand a cat in hell's chance at SCC.
The mis-description is down to you. You are supposed to be the 'professional' and should check your listings carefully. If you make a mistake you have to take it on the chin. The broken door handle is down to them.
A seven year old 500 auto is a grenade with the pin out. Those robotised manual boxes are a nightmare. Many didn't even work properly from the factory. I wouldn't want it back and I'd be keeping my fingers crossed for 6 months.
My advice is try to negotiate some sort of deal with the customer. Say £250 compensation for no CC and send them a handle to be fitted at their expense.