This is the other endpoint problem - this is a zoom in on the upper section:



The line and the upper part had the same endpoint on one side which allowed them to be joined into one curve at that spot, but the other ends don't line up, the line is shooting off past the other end a bit.

So to clean this up, I used Edit/Separate, then used Transform/Scale/Scale1D to stretch the upper part so that it matched the line underneath it, then rejoined them. This type of tuneup was needed on the middle and top sections (result .zip attached).

With those cleaned up, it should now sweep into a capped solid like you want.

- Michael