Had a bit more time and it's not quite so hot. So was able to drill that hold in the stringer and have set up another configuration that I am hoping will work.
The vertical 1.5" PVC will be connected to the copper but I didn't want to cut that piece just yet.
Here's a better angle showing the trap. We're thinking of trying to get a small 22.5deg connector in there to produce a good down angle leaving the trap. Otherwise I think I can tweak it during the gluing process.
Here's a wide angle showing the drain and where it connects to CI. Have to go back to Ferguson as they gave me the wrong Proflex to connect 1.5"PVC to 1.5" CO.
This question is not about the tub, but the toilet, which will be located very close because we didn't want to mess with removing that CI monster.
I found this CI offset toilet ring, which supposedly has a rubber gasket to attach it to 4" CI pipe (I'm fresh out of packing and lead).
Obviously some of the vertical 4" CI pipe will have to be cut off, but the question is, will this work? Looking down into it, it doesn't look like it will drain properly (the plastic cup will be removed).
Using this offset will give us a couple more inches between the side of the bowl and the tub, which right now will be about 12 inches. That 12" doesn't account for the frame around the tub and any tile.