"if the pads are new OEM parts, no shim is necessary since that feature is already incorporated."
Be careful with the word OEM. I'm not positive but it is my understanding that OEM is not the same as Volvo. You can search the archives on this site for that topic. To solve my rear brake problem I used new Volvo pads AND the Volvo cut-away shim. I don't think new Volvo pads has a built in CUTAWAY shim.

"how bad do you think it is to live with it for another couple of months? Others had to live with the problem for several months with no dire consequences, so I'm guessing it's ok."
The only reason I lived with it is because my wife drives the car :) She complained every few days and I "quickly" solved the problem - a few months later! I drove it once in the mountains and thought I was going to die. The horrible noise and vibration was constant with every downhill braking.

Bottom line in $ was about $140 for two rotors, $35 for pads, $20 for shims and silicon grease. I did the labor. After all the messing around I did to discover the true fix, I would have willingly paid the dealer the $500 or so they want for rear brakers and rotors!