Fantastic advice! I only end up with the 'collapsing to a point' challenging geometry because I generated the sharp carved out part with an extrusion, bool, and then subsequent addition of a curve (valley center) and surface generation from there. That leads to 'sharp' edges. It did not occur to me that I can use another surface to trim the existing surface, thus creating already the valley profile I actually want..... From there, controlling the 'rim' of the valley is much simpler and straight forward....
Thanks a lot for your comments!
Lemo