How can I join polylines into a single one that intersects?
You’re going to have to clean up stuff, but you could try patching the poly curves into a surface, and then joining the surfaces into one, before pulling the perimeter curves.
Those discrepancies in the internal points which don’t close may be an issue though.
Need to see what nodes you used, but my guess is you are using an incorrect method for patching the surface.
That node I am using.
Is there a way to know which point is inside the polyline without making a surface?
Not sure. I won’t be at a CPU until Monday, but I haven’t seen patching like that before. Try patching each PolyCurve individually to see where the failure is. My guess is you have a bit of bad data in there.