To solve the problem above, you can use something like “ruled surface”.
This will ensure a single untrimmed surface in GH, but a brep once baked in Rhino.
Also, the curve control point weight is not constant 1 in the bottom curve, because there are arcs involved, but the top curve has the same weight as 1, this will cause some problems in the surface structure as you can see in Rhino, and the pic below
So this is the final approach with some special treatment to knots and weights.
There are some geometry tricks dealing with control points and plane alignment.
If curve A’s all control points C are on plane B, the entire curve A is on plane B, whatever plane B is.