Hi Lara, please see the attached 3DM version for a replacement for your red surface.

The way I made it was not by directly lofting between that upper and lower edge but instead extruding this one downwards:



So that makes a very regular shaped and extended surface like this:



Then trim it with this to slice the bottom chunk off:



So often with NURBS modeling you don't want to try and build a surface that directly hugs along some irregular shaped boundary, it's often better to build a simplified and extended surface sheet which then gets the irregular boundary applied to it by a trim or boolean cutting operation. That's the main way to get higher quality surfaces, when you trim a surface the surface shape stays the same and it just gets new trimming boundaries on it marking which areas of the surface are active.

Hope that makes sense!

- Michael