I am having the plumbing for a second story bathroom installed and there is an issue. This new bathroom will be on the back of an existing bathroom, so everything is being tied into together. The issue is the toilet on the other side has this configuration (90, 1 foot piece of 3 inch, 3 inch tee with the vertical being the main vent stack, 90 turned down to go down inside the wall). When the floors were opened up, the ceiling below on the side where we want to put the new bathroom is to high the get below the 90 that drops down for the main drain. So we cannot tie into the main line with that tight of a configuration, and we do not want to open up the ceiling in the dining room and make a false wall or box down there. There was a thought of putting a back outlet floor mounted toilet in and tying into the main vent stack. The issues I thought of with that are that if we tie into the vertical vent line (which would be 6 inches above where the 3 inch line for the other toilet is running) when the toilet flushes, it will fall vertically down into the bottom of the tee then flow gradually down thru the 90 and beyond. I'm not sure if it's a good idea to have you're toilet fall into the bottom of a tee, seems like a good spot for solids to collect and clog you're drain. So we are in a spot where we don't have enough room to put a floor mounted toilet in because the logjam of fittings and no room for access to the pipes, and the wall outlet toilet might be unwise to use with the drain falling into the bottom of a tee. Just curious if anyone has any ideas or suggestions. Thanks

You turn down in the wall next to your main line and tie in below the elbow.

Any suggestions on how to cut below the elbow? It's about 8 inches below the floor in a tight spot between the walls. Also, could I replace the whole conglomeration and put in a double eye with 1/8 bend then two street 45 out of the top to reconnect with existing vent?

Can you post photos and more details with an Isometric drawing. This will help us to help you.