I have to say that trimming in Rhino is about as infuriating as filleting. People always come with comments like oh, that curve isn’t exactly that or that curve isn’t this, but I don’t think I should have to care, just like you can just “fillet-and-forget” in most other CAD software, you should be able to “trim-and-forget”, but no it’s super finicky and manual like everything else that gives you RSI in Rhino.
What did you think Auto-cplane was supposed to do to help here? No one has ever actually used that feature on purpose.
#2 split fine if it was exploded, which is expected. Splitting polysurfaces with curves in 3D only works if there is no pulling required. #1 splits fine in a parallel view. In the perspective view I need to Pull on to the surface first which it is supposed to do itself. Maybe the input curve, which is mostly very close to the surface, is close enough that its check of whether to pull or not fails, I dunno… yeah if I move it another mm or 2 away from the surface it works.