Can I propose we flip your question on its head?

Sure. Why not.

Grasshopper as a panacea to reduce rework of a model?

Yes, mostly. I am not bright enough to be able to remember the path through the maze 30 days ago, or even 2 days for that matter. But GH is a parametric tool. Once I have summited this mountain, there are more. a structure must be built to stiffen this “bathtub”, and once that is done, we have to see if what remains is big enough. If not, some sliders will be moved.

it’d be handy to know how the object is being manufactured

NOT injection moulded, welded sheet metal of some sort probably, so next is a thread on panneling challenges. Brace yourselves )

your quest is fairly nebulous at the moment.

Well yes, and deliberately so. Diving into specifics doesnt really help the big picture. Like for example this little chiken and egg surfacing problem:

The intersection of these two surfaces produces a curve. That curve is the tangent transition along the two surfaces BUT, the red surface was created without a vertical curvature that the finished surface will have, so the curve will change. The red surface is a lofted surface from two curves. It needs three curves or more, but to make a curve in between the intersection of these two surfaces is required… another iteration problem that must be approximated somehow.

If I use the curve generated above, then when the red surface is lofted using the now completed set of vertical curves, it will have an artifact resulting from the innacuracy.

I dont even know if its possible to curve the red surface using a curve, an arc in the vertical direction. My current idea is to use NETWORK SURFACE and thus include the arc to derive that curve (which will be different)

Regardless of the model, the approach you take to modeling is crucial. I began thinking that I could just trim away and join surfaces to get a perfect result. I have abandoned that approach. I now use the trimmed surfaces to generate curves from which to loft one surface that forms the entire side of the bathtub excluding the bottom which I hope to join with a fillet along the length.