With regard to the MDA it is a legal requirement for us... As it is in the Operational Manual as approved by the NAA.
We have to add 50 feet to any MDA, as it is not permissible to go below the MDA I don't see how you can assure you don't technically bust the regulation without an additive when performing a CANPA approach.
When the approach is designed (current me if I'm wrong) a DA has an allowance due to the fact a go around at DA will result in the aircraft dropping below the published altitude.
An MDA however, there is no factor built in.