Glad to see you're enjoying the TBM so thoroughly! This is exactly the kind of investment into one of my aircraft that they were built for. To answer your questions quickly in bullet from:
The inspection time will reset at the proper intervals.
The "key %used" refers to the USB stick in the unit that records all the data. It will also reset before hitting 100%. It's fun for me that you've flown enough to notice these details!
The TBM should already remember fuel load between flights, as all my aircraft have the code to do so, but state saving has been intermittent for some users since MSFS 2020, and sometimes bugged completely in MSFS 2024.
You can absolutely use the default payload interface instead of the tablet! That's actually in the FAQ in my manuals. I haven't checked in a while, but my canard aircraft might flip over backwards if you set passenger payload with the default payload interface, due to a lack of foresight on the part of someone at Asobo. We've asked them to fix this, but I don't think it's happened yet.
Looking at the code, I'm not sure what L:Var you're using for the propeller heat, because I didn't create one. It should be as simple as using the native event: >K:TOGGLE_PROPELLER_DEICE.
Are you sure the button animates with the L:Var? If it does, then the sound should play, since it's tied to the animation. If the button does not animate, then you might try adding _IsDown to the end of the L:Vars, which works for some momentary switches and buttons.
