I've owned a few of the PC 7424 & 7424XP over the years and have never dealt with any of the shaking/noises you mentioned.
In your case it sounds like it's the motor. Could be wiring, but it's obviously only happening once it gets warmed up.
Time to do some shopping. Cheapest I've found is...
This might help you. It pertains to cloth interior, but in this case the scenario fits your situation as well.
How to Remove Oil Based Paint from Upholstery | Cleaning Guides
Great work!
The scratches can be a pain, no doubt. I typically will continue to fill the scratch with touch up paint until it is level with the clear coat. Then I'll begin sanding and buffing. Usually it'll finish nearly flawless, but not always.
There's a detailer I know of in Austin, TX whose company is "Meticulous Details".
Small world .....
By the way, I had once paid for a logo to be designed and I never truly liked the logo better than what I came up with. It was just $150 but still ...........