Sorry for the slow reply.
I think my confusion was the inconsistency in behavior between scissors and erase. In scissors, there is an “Apply to all segments” checkbox, which is actually “apply to all visible segments”. I thought (incorrectly) that the “Erase all segments” checkbox would behave similarly, but it does exactly what it says it does - erase all segments, visible or not. The tooltips for both checkboxes (which I had turned off, unfortunately) do document the appropriate behavior correctly.
Erase all segments does work correctly with Editable area set appropriately, I do not see any issue with that.
So I guess this is a case of “nothing to see here, move along…”
Thanks!