Steve,
Thanks for a great writeup of your odyssey! It's good to read that it has worked out so well.
As a structural engineer myself, I found your story of particular interest. But I think you have a typo... I suspect that the new floor is designed for 150 pounds per square foot, not 1500. (The normal residential design is for 40 psf.)
Yes, that makes sense...i think even industrial warehouses with large-loading dock bays for 18-wheeler trucks are built for 300+ pounds psf load.