RE: Lofting Control
When I worked through mine, I surface modeled the outer shape with 2 profiles (entry/exit) and 4 rails (top, bottom, left, and right).
To avoid the surface ‘pinching’ near the chainring clearance, it would be more controllable to overbuild the overall shape and then cut away the necessary clearance for the chainring.
For everything else, I used some guidelines in this thread: Need 3D printed pipe fitting? $1/g printed Monthly
But I’ll let the more experienced folks chime in.