No wiring diagram for the Volt on the Internet? Even just the BCM input Shift to Park switch?
Cars are built to be manufactured, not repaired. Simply replacing the microswitch is not can option because it's an entire subassembly 84955381 of the shifter - the BTSI solenoid, wiring harness, connector are all a prefab piece. Suppliers having a lot of extra stock of that, or the entire shifter assembly who knows what was agreed to.
For those that can solder, it's no biggie to replace the switch but that does nothing for the root cause, back-EMF spikes.
I would say it's two problems - the vehicle trashing the microswitch with overcurrent or arcing or both, and a now dead microswitch. But "Shift to Park" switch failures are common - in Ford Edge, Cadillac STS etc. many other cars too.
The GM fix looks wrong to me - my guess is the solenoid(s) are in series with the microswitch, so the back-EMF trashes its contacts. You'd have to do an autopsy on one to see the contact failure.
There should be components at the solenoid(s) to quench the back-EMF, but as a diode this ruins the "reverse battery" automotive requirement so I see some cars missing it. Even a 1k resistor across a 12V solenoid, or RC snubber lowers that spike well enough. But the costs... too much.
Some (GM service bulletin) car models the park switch is on the high-side, others low-side to GND. The universal "fix" might not make sense for the Volt as it switches to GND.
On my old '60s Chevy the Park Switch was in series with the ignition switch and starter solenoid, so it saw 40A during cranking

Better modern cars run the switch only into a BCM digital input.