I lagged on replacing my rear brakes (light came on a year ago) thats why I'm down to the metal. I can hear the grinding everytime I hit the brakes, but only the passenger side. The driver's side rear pads have at least 1.5 to 2mm. Does anyone know what could be causing this or do I need not to worry? Also the rear rim on the driver's side is a little warped, causing slight vibration. So I've noticed there is rattling from the rear tire whenever in motion or going over bumps. Mechanic says that its from faulty aftermarket pads. Lastly, parking brake is no longer effective. Any feedback would be greatly appreciated.

A millimeter or so difference in pad wear isn't something that I'd get too excited about. But it could be that the caliper piston on the passenger's side is binding just a bit. The parking brake problem could be from misadjusted or worn out shoes, cables, or broken parts. You'll have the chance to inspect the parking brake and adjust the shoes and/or cables when you do the brakes because new rotors are going to be required.