Sealant on the rear bottom plastic piece
 

I've got some Wolfgang Sealant. Is it safe to clay bar and put sealant on that bottom plastic piece?

i wouldn't
keep it to your paint, headlights, and tail lights

Typically, trim, plastic and emblems/badges are taped off when applying wax or sealant to prevent getting residue on them. You can clay it but I wouldn't put sealant on it.

For that piece, you want to use something like 303 aerospace protectant.

i've got some carpro dlux. how would that work on that plastic piece?