Plumbers....adding a shower drain questions

Remove the plumbing back to the existing and dig down about a foot so you aren't fighting yourself. Snap lines to locate your new drain location. From the existing plumbing go down with a 45 to another 45(this forms one end of your trap). Run a long length (more volume to keep the trap wet) to a sweep 90 up to your drain location. Don't use the dwv fittings with the shallow glue socket. Pressure fittings have 1 1/4" glue sockets. Dry fit it all together before glueing. You've now created whatever elevation change you need, made it easy to snake, and given yourself more protection against sucking the trap dry.

I'm assuming that thing with the furncos is the check valve. Go return that, and get your money back. Never install a valve where it can't be serviced. That thing will actually work as a snare and won't prevent backflow once it doesn't seat.

I gotta run around for a while, but I'll expand on this later tonight when I get home.