You had better use some thought if you start cutting that much concrete slab away from the outer wall, as the slab is the only thing holding the bottom of the wall on the footing. When the ground gets saturated with water outside, that wall can get pushed inward off the footing and collapse. There are a lot of depending factors, but it does happen and it is a very bad thing.
As for the plumbing, I would run a main line from the toilet to the clay pipe and then run branch drains for the lav and shower at 90 degree angles into the new main line, attaching with combos. The lav drain and vent stack will come up inside the wall. The shower vent will come up inside the wall of the shower alcove.
The new main will need to attach to the clay with a wye also. I have never worked with clay, nor do I ever hope to, so I cannot help you there.