Realistically, I was in your situation, I'd probably:
1) Phone up HR and bitch up a storm about their screw-ups, and that they've gotten everything they're going to get from me.
2) Wait for a demand letter from them
3) Get them to document how they calculated the overpayment (basically drag it out as long as possible)
4) Wait for the "Pay or we'll sue" letter (maybe, or maybe I'd just got to step 5. Depends how p1ssed off I was still)
5) Pay the buggers and get them out of my life
But if you're operating under the premise that you'd win if you went to court, I'm thinking you'll be out of luck. That's all I'm saying.
ETA: I'd also rip them a new one about the deductions that they weren't allowed to take off my last paycheck (when I talked to HR in step 1), and tell them that in exchange for not taking them to the Employment Standards people, they can just get out of my life.