Will a MB dealer sell a CPO warranty to a MB owner, even if the car is:

1) still under original factory warranty (4y/50k)

Yes, the CPO gets added on to warranty and kicks in after the factory warranty ends.

2)vehicle was purchased from a private party

No way no how. The vehicle has to be inspected and certified and then RETAILED at the MB dealership.
Now if you can talk the dealership into buying it and then selling it to you......