The opening in my CAD contains diagonal lines, so when I group curver and create shaft opening from them it gives error. Only lines created from circles or openings in cad without diagonal lines can be created.
Convex hull should account for most cases, but if not you could take each curve and check how many other curves it meets. In total a diagonal should meet 4 other curves, whilst a non diagonal edge should meet 3 or less.
Assuming you don’t have any curved lines… worry not about the curves but their endpoints.
Any line which is on the diagonal will have both endpoints touch a total of three curves. Any line in the perimeter will always have one end which only touches two curves. Testing endpoints for intersection counts should allow you to build a boolean which can filter to just the diagonal.