a) too much entry speed is usually the reason, on a tight course like streets, for understeer. If your friend was able to knock 2 seconds off your time, I think that's probably a big part of it. Especially running with MDM on and overheating the brakes. That tells me the car was struggling mightily to save you from yourself
b) that seems like a lot of rear camber, necessary to fit the tire? If not, stand up the rear tires a bit. I'm assuming you're using an 11" wheel in back, if so, 315's on an 11 are still a bit pinched and you're going to roll over a bit, plus the outside rear takes a lot of load but it's transient, it's not like the fronts, a little rollover isn't going to cord the outsides of the rears. This sounds like what the 295/315 hoosier setup I had did, the rears looked like they were rolling over more than the fronts from the wear marks, but the fronts still wore the outside shoulders faster because you grind on them harder in corners whereas the rears are only rolling all the way out for a short time during traction events. Different tires and setups, but keep that in mind when interpreting wear marks, they mean different things on different axles
c) Sounds like you need to drop front pressure a bit too, seems like a lot of front camber too, those two are interrelated. I understand from others the 615K+ likes pretty low pressures, but, grain of salt I've never run them