The plastic parts should not need a clay bar, as the clay bar is designed for pulling contaminants out of paint and not plastic. I don't think the plastic will pick up any contaminants anyway, so no harm will be done even if you do go over the plastic.

When using a buffer, I find just the machine's weight too light. Put moderate pressure on the buffer, but not too much so the machine wants to bog and slow down. You can use a buffer to polish A pillars with a smaller pad and backing plate or it would be too clunky. Doing it by hand is also an option, but the reason for a machine to do this job is the even pressure applied throughout the process.

Lighting with halogen work lamps and a hand held fluorescent light would be fine for looking for swirls. A well lit area would help too, so it isn't only your work lights guiding you.

__________________