We had a second floor put on our house and in doing this, they built the walls for two bathrooms and roughed in the plumbing for one of them. I started preparing to build out these bathrooms and seen the cut the main stack just before it went into the second floor and put a 90 on it with a curve then a straight and then a T and then they put it up through a wall on the second floor and through the ceiling. the main stack was a 4" main and now it is up to the 90 - it is a 3inch from there to the T from the T up through the roof it is a 2".

Question 1: is a main stack allowed to go from a 4" to a smaller diameter
Question 2: is a main stack allowed to be cut and relocated like that?
Question 3: where they have the 90 and the curve - is that allowed or should
that be a sweeping connection

Thanks in advance for all your help !!