Because it is not cast iron, I highly recommend the mortar base. The tub will not feel solid without it. The mortar is placed in small piles on the subfloor and the tub is set or rolled down into the mortar bed.
The Kohler instructions are pretty clear about the framing that is required around the tub. A tub is normally set on the subfloor and the floor tiles are cut to fit up to the edge of the tub.
There should be insulation in any outside walls. Most of us use what is readily available. Spray foam is the best, but not many companies would be willing to do such a small job. It would take more time to set up and clean up than it would to apply the insulation.