There is a hard truth to be learned here. Imported curves generally suck in fusion. sometimes it's because they where created poorly, sometimes it's because fusion's import mangles them, sometimes a combination of both.
seriously, this comes up so often here I'm thinking of selling t-shirts that say "it's the imported curves".
I understand the desire to create beautiful curve geometry in other software and bring them into fusion, since sketching in fusion is sometimes a but of a chore. But frequently there is so much back end work required to get these curves to function right that it ends up not being worth it. This is a work flow that many beginners try. B/c of the knowledge necessary to be consistently successful with these curves , I would consider this an intermediate workflow, not a beginners work flow.
The issue with your imported curves is probably the most common one. lines that are almost tangent, but not quite (near tangency condition). This eats Fusions lunch.
here is a pic of tangency combs on part of your curve showing this-

best advice is to redraw the curves with native fusion sketch and use that for geometry creation.
there may be other geometry issues that come up at that point, but one battle at a time.