Can I damage anything in my shack by feeding PSKMETER a signal which is different from the expected PSK31 signal?
No. The PSKMETER has a front-end zener that protects the ADC and microprocessor from over-voltages, preventing damage to the PSKMETER. Since the meter is just a monitor, you cannot damage your transceiver. Similarly, the Windows software controls only the output level of your sound card and cannot damage your computer, its files, the sound card, or your transceiver.