quote; 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
1. Usually yes, but the individual situation would determine whether it is proper or not.
2. Again normally Yes, but exactly how they did it could be a factor.
3. A sweeping connection going up, a "short radius" going down is acceptable.
NOW, the big question is; WAS that pipe originally the "vent" for the downstairs bath, and if so, is it still the vent? If yes, then NOTHING they did may be proper or approved. You are asking questions that should have been addressed by the plumbing inspector when he approved it.