The only mistake I see that you made was letting a Carmax (work's on everything, expert in nothing) mechanic work on a specialized car like your MINI. Makes you wonder what else they fixed that's going to come back on you later, doesn't it? I know they were "fixing" it for free, but what has it "cost" you to NOT have your car for a month? You're still making the payments, right?
Chances are there's nothing wrong with the car you bought that can't be straightened out, and once fixed it should be a great car for you, so you can have confidence in that at least.
Next time, I'd suggest you take it to someone you trust, whether Baron (my 1st choice) or an independent you've used and have confidence in. These are expensive cars, and easy to screw up, just remember that for the future. Oh, and your MINI dealer warrants all of their shop work - parts and labor - for 2 years......no one else does.......no one.
You also can come to some of our club's shop days and learn some things about your car if you want to do some of the routine stuff yourself, we'd be happy to show you some good tips and easy maintenance things you can do.
You've learned a valuable lesson here, grasshoppa.......the cheapest price is not always the best deal.
BTW, I really like your hood stripes!