Not sure of the exact error(s), but the first problem that I see is the car wash process itself.
The youtubers are so busy trying to promote products they'll use 2 or 3 different chemicals just...
I've never used any of the products that you mention, but from experiencing simular problems I would suggest you polish for a longer period of time. Polish until there's hardly any residue.
I would...
The black specks are a sign that your backing plate is disintegrating. A wise man would change out the plate when they start to notice this happening.
Yes it's happened to me
I wouldn't call myself a rare breed. My car looks like a b o m b went off in it 95% of the time. It also has a very funky smell from wet dogs and spilt chemicals.
Drivers fender has definitely been repainted. The hood needs compounding. If the owner believes it's a creampuff I would go ahead and let him/her keep thinking that.
You'll be fine. Try not to get the car real dirty between now and Wednesday. When you're ready wash the vehicle using nothing but water. I wouldn't bother...