Hi koi, so for the bottom not being flat that's partly because your profile curve does not have an ending tangent that points straight downwards, it's angled like this:


So therefore the surface normal that a surface offset will go along will be like this:




I'm surprised that shell actually worked here because the surface you've generated is very strained and badly formed in the area where it collapses down to a point:




I'm not sure what your goal is for this protruding feature as it traverses towards the tip:



But you probably can't get a well formed result by just having that protruding feature directly collapse down to a point all on its own. You'll probably need some additional stations set up so that there isn't a complex shape with near undercuts in it as it collapses down. Something like this using Construct > Network (see attached 3dm) :





And it may be better to directly model the inner surface with its own set of curves rather than generating it with a surface offset.

- MIchael