Can I use creme fraich and/or buttermilk for sour cream--would you use half of each?

Crème fraiche is essentially French sour cream so sure you could substitute that. Or even Mexican crema. I doubt that buttermilk would be a success.

And PS: i usually don't buy mascarpone for a cheesecake either. Perhaps the texture might be a little creamier, but I have no problem with the texture using just cream cheese.

I think you can substitute sour cream for creme fraich, but I wouldn't use buttermilk. The texture would be wrong as sour cream is thicker. You won't notice the difference in flavor.