Soapy water isnt bad for plants at all. It really only will effect aphids and insects. So I kinda see it being beneficial. But yeah keep in mind if you use some acidic cleaners or a strong APC I imagine that may have some negative effects.
When I worked in a body shop I used to wear my buckle or belt to the side. Then I picked up a canvas velcro belt. they come in a lot of different colors.
Yeah hand polishing is always okay. It's much safer for some spots.
Keep in mind when using UC that you want to make sure you work it until it starts to become almost clear. It has diminishing abrasives which breakdown and polish finer the longer its worked. That will prevent hazing and...
Of course. Its generally recommended by the manufacturer that you stick to their line. There is nothing wrong with mixing though. I just used UC and followed with wg polish and my results were very impressive