I would like to get your recommendations on troubleshooting and fixing main drainpipe leakage and possible washer standpipe design issue(s). Background: One of our younger relatives remodeled our laundry room in our Salt Lake City, UT basement a couple of years ago and we have had some issues with our plumbing. About a year ago, we had flooding coming from behind the wall while the washer was running. Upon further inspection, the issue was that the standpipe was not glued into place at the base. I thought the problems were resolved but I have noticed some minor leaking along the baseboards. The standpipe spans the basement to the kitchen where the kitchen sink and dishwasher are connected. I am unsure if the main drainpipe is vented; I did not see a vent to the roof. I cut the drywall open and have some concerns about the design and how it was put together. The issue appears to be sludge-like material seeping from where the 2” waste pipe connects into the 3” waste pipe near the base of the floor (see attached pictures). I am not sure if the issue is a result of the plumbing design, the quality of the workmanship (gluing/connection issues), if there is a blockage in the lines to the sewer, venting, or a combination of these issues. Based on my research, the washing machine standpipe height (currently 9”) and p-trap distance from the ground (currently 36”) appear to be out of code. There is also a pipe that connects to the base of the 3” drainpipe (see pictures) that has been reconnected to the upper portion of the 2” drainpipe just below the piping to the washer; I believe this used to be a pipe that connected the to the old washer standpipe (before the remodel). I have also included a diagram of the situation (see pictures). I opened the drainpipe access and it appears to be clear (there is sludge on the backside of the cap). The other plumbing throughout the house appears to work without issue. My current approach to fix this is to replace the 3” pipe just below drain cover and extend up into where the 2” pipe, get rid of the side pipe that starts & reconnects to the main drainpipe, and install the washer p-trap within 18” of the floor to the 3” pipe. I also plan to flush enzymes through the main drain pipe if there are blockages with the sewer line. Thoughts?