I copied it into a new scenario and I'm remeshing. There's something interesting with the meshing engine, because the first time I tried to remesh, one of the parts failed during tet generation and reverted to using the "old method." Three days later, it was still meshing. I killed it. Then, when meshing again, another part failed during the solid meshing, "failed to solid mesh and/or was not watertight." This part was just successfully meshed in the previous attempt.
Is the meshing engine non-determinant? Is there a random seed generated within that would result in different different results each time I mesh?
So the solution would not mesh in Design Scenario 2 using the same parameters as in Scenario 1. I had to reduce the mesh size of one of the parts to avoid that tet mesh failure using old method error. Once I did this, the solution meshed completely.
The internal mesh is visible outside of the part again, similar to, if not exactly the same as, Scenario 1.