Actually i don't know if it even shoul'd be on the curve. (But i am a starter…and i don't know many nodes yet.)
Anyway here is your Scene file re-saved as *.hipnc(sorry only Apprentice here) with one added switch node, state 0 is a grid(a face) extruded along curve1(through a sweep node and some other nodes(inside the subnet).

Hope this helps somewhat.

Yep modeling operations are handled in the operator stack and undo system, selections undo.

I did a similar mistake, i messed up one of my PolygonSplit operations, the result was the same, the collision didn't work properly on the column.
I backtracked every node in the column tree with bypassing them until i found my bad node…

Maybe i could just have rebuilt the entire thing faster, but i learned something from it.