Original model contains weirdness where shapes intersect, like there are triangles inside the model. It make Simplify3D leave out the overlapping parts creating hollow spots inside the model. It was unstable and fell apart after printing for several hours.

I imported the original stl into tinkercad, upped it to 150% size and exported. This seems to have gotten rid of the weirdness. Printed first try without falling apart half way through.