Hey Nomuse,

 Well, there has been a bit of a paradigm shift in the way C5 models compared to C4 and it looks like you don't like it. The Vm in C5 does have its bugs but it is still a very capable modeler.
  Basically you can't connect 2 polylines using the "Connect" or "G" hotkey anymore, you will get that "cannot add edge in the middle of polyline mesh" error which is the one I think you meant in your post.  What you can try to do is Dynamically extrude each of the polylines in the same plane and then connect the 2 polymeshes  with the "G" hotkey to form your new mesh. You can connect two polymeshes with the connect tool just fine.
  So , I would do the following.  After you duplicated,flipped and welded your curves together, in Plan view(from top) I would select the outside curve dymanically extrude it (H hotkey).  Do the same for the other 2 curves and then connect them all together with the G tool . This should work for you and good luck with it. 

Mike