The flipper had ignored all building codes and created a plumbing nightmare, with a piecemeal zig-zag of improper unions, a covered floor drain, backwards Y-unions, pipes cemented into the cleanout, and no drain vents whatsoever.
Today, my plumber laid new underground drain pipes and properly connects them to the main sewer. He includes a new cleanout, a new floor drain in an accessible location, and new vents for the entire system.
Building code says that a joist becomes too weak if it is cut more than 1/3rd of the way through, and ideally the hole should be in the center of the joist. In this case, the upstairs bathtub drain pipe needed to go through a joist, but the flipper cut away 2/3rds of the joist, severely weakening it!
I repaired the weakened joist by reinforcing it on both sides with plywood. Each piece had to be carefully cut to fit exactly around the pipes and provide as much structure as possible.
Earlier, the plumber needed to stop the leaks inside the walls of the upstairs bathroom. He did this by cutting some access holes on the backside of the wall, which happened to be inside of a linen closet.
I made new cover panels for these plumbing access holes inside the linen closet. I also gave the closet a fresh coat of paint and re-lined the shelves. ($37.26)