A. All whitening systems will fail to lighten existing porcelain shades. If you have a mix of crowns & natural teeth the normal procedure is to lighten the natural teeth to a stable shade with a decent system. Then replace the crown with porcelain to match the stabilised lighter shade.
The whitening is cosmetic and the fee would not normally be covered by dental insurance, if the crown is found to be defective the replacement fee would be covered.
Some low cost schemes such as BHSF, HSA crown plan, just ask for a receipt and dont exclude any particular treatment but limit the claim value to a few hundred pounds a year.