Did you try the old unplug and hold the reset button on the back for like 30 seconds? I read on a forum that a purple screen usually means the stick lost its firmware and a hard reset can sometimes bring it back. The trick is you have to hold the button while plugging the power back in, then keep holding until the light starts blinking. Mine did the same thing last year and that actually saved it, so it might be worth a shot before you drop money on a new one.

Lug nuts back off on their own sometimes. Heat cycles, cheap studs, aftermarket wheels, all kinds of stuff. Two loose ones out of five is not the same as the shop leaving them all finger tight. If they really forgot, you would have lost the wheel or at least felt a bad shimmy on the highway, not just a clunk in a parking lot. Twenty minutes of re-torquing is annoying but that is a normal thing to have happen. Get a torque wrench and check them yourself once a month if you are worried.