Hi airdog, this is the method that I would recommend for the "trim a single surface" approach.

If you are starting with a surface that has a bump in it:



Switch an elevation view, like in this case the front view. Then draw a closed profile curve. In this case I drew the curve in 2 pieces, kind of following along the contour of the surface, then I selected the 2 pieces and used Edit/Join to glue them together, the result looks like this:



Now to trim the hole, make just the surface selected, then run Edit/Trim. Then MoI will show a prompt in the upper-right corner that says "Select cutting objects", at this point select the curve, it should look like this now:



Push "Done" or right-click to tell MoI that you are finished picking cutting objects. Now the surface will get sliced up by the projection of the curve:



The prompt will now read "Select pieces to remove or push Done to keep all". You can now select a fragment to remove, like this:



And then click Done or right-click in the viewport to tell MoI you are finished picking pieces to remove, and this is the final result:



So I hope that might work as a way to do the trimming that you were asking about originally.

A bit later on today I will see if I can give a different example about building your thing in strips instead of from a single lumpy surface.

- Michael