I wanted to make a shell of my partially reverse engineered shoe last.
It's only partially reverse engineered because I only need the lower region to slice off the top, the remaining part would be 3d printed to be an upper part of a shoe sole.

The model consists of the following items, from left to right :
toe cap/first part from left is sweep, 2nd is sweep, 3rd is loft in x direction, heel is loft in z

Here is what I've tried so far :

1st: I've created a single surface, I used inward shell on each part, The problem is that I get good solids but also microgaps where the parts join and that messes up the 3d print, I get an ugly weld at the place of the gaps . I've tried many things to join/union the surface and shell as one piece but nothing worked so far .
The other problem with the shell, is that the toe cap, the first part from the left ended up with some really abnormal extra parts .

2nd
I took the time to offset all the curves I use manually one by one certain ones by copypasting and moving manually, then I made the shell manually, the best I got out of this is joined surfaces, but I'm unable to turn them into solids .

I could really use some help :)