As a follow-up to this technique I've attached a simplification because although I could create planar on one side after following your technique Michael as I was unable to reproduce on the other side. A mirror does work, but planar or the following technique does not. I extruded a line vertically from a magenta curve on one side and a cyan curve on the other. The magenta curve can be boolean diffed or trimmed with the "curiosity" object without issue while cyan curve cannot do either. I'm unable to see any difference between the two sides and reproduced in a symmetrical manner so I'm stumped ?
Furthermore when viewing from left side I should see cyan curve and from right side magenta curve, but I see magenta from both sides.

Sven