I've not seen reports of anyone doing long term experiments to really answer either of those questions 100%.

The general consensus seems to be that the amount of white in pieds is random but it could have a genetic component that is just more complex than we've figured out yet.

Also with pastel the general consensus is that they are all the same gene (except for enchi and cinnamon both of which once had "pastel" in their names). However, I don't know if anyone has say crossed a lemon super pastel to a Graziani super pastel and confirmed that the resulting animal not only looked like a super pastel but also could produce no normals. Assuming pastel's are all the same gene the even harder to answer question is if they are all exactly the same mutation of that gene with differences in lines all being selective breeding to add other enhancing genes or if there could be some actual variation in the mutation of what we group together as pastel.