You don't have to pretend to be him. It's your Mac Mini. You paid for it, so it's your serial number now. He's stolen it from you. Apple is harbouring a felon by not revealing the name of the man who now has your stolen Mac Mini in his possession.
You need to make that very clear to Apple that the name and address they have listed for this serial number is for the man who stole it from you. Also report the serial number as stolen to the police. Don't bother with the 'con job' angle. It was straight out theft. Just tell them he swiped it from you while you were at the gas station, which is the truth. Let him argue differently after they arrest/charge him.
Edit. I personally don't see any difference between him swiping it by switching it when you weren't looking, or simply swiping it from the back seat of your car when you weren't looking. Same thing. He swiped it. Theft is theft and is a crime, not a 'civil matter'. He is in possession of stolen property.
I thought this was a good idea at first. But there's a problem. Apple's records would show the mini as registered to that guy for months--which wouldn't be the case if he just stole it and then called to switch the address. So in court it would just look like the OP claimed 'his' Mini was stolen (I assume he doesn't have any record of paying for it), while the scammer would say Apple can verify it's always been his and the OP is lying.