Hi Chris, I've attached here a 3DM file with a fixed up version that's a solid.

To fix it up, in that one area with duplicated stacked surfaces I deleted the stacked up faces there, and also did an untrim of the surface to the right there that had a triangular hole in it, something is wrong with the current boundary there. To do the untrim I used Edit > Separate on the face to break it out into its own individual surface, then selected all edges (select one edge and use Ctrl+A to get them all easily), and then hit delete to remove the current boundary and have the underlying surface's own natural boundary restored there.

Sometimes when you untrim something, the underlying surface can be a fair bit bigger than the original boundary so you may need to retrim it, but in this case it matched up fine so no retrimming was needed. Then I rejoined that untrimmed piece to the others, and the remaining boundary was close enough to planar that selecting the 4 edges and running Construct > Planar built a surface there which I joined in and that finished up that particular area. Some of the pieces in this particular area of the model are not really aligning very exactly with one another but they are close enough that it should be ok for them to be joined. But it helps in general to have surfaces coming together more accurately.

Then in the other area where the was some overlap I deleted the 2 surfaces that were overlapping a little, did another separate and untrim on the surrounding surfaces so that I could see the base surface structure, then I drew in a vertical line in the middle area, and did a sweep selecting the fillet arcs as the profiles, and an edge on the left and the vertical line on the right as the 2 rails. Then the remaining area was close enough to planar to be filled in with Construct > Planar.

For the 3rd area with naked edges it was enough to just do a separate and rejoin of that particular spot.

With those repairs in place the model is now all connected and makes a solid.

Hope this helps!

- Michael