>First question, why is there no slot down the side of the frames to keep the foundation stable?
To save wood and cuts.
>Second question, why are the sides of the frame narrowed by 1/8th inch at the bottom?
For better communication between frames. They can go around the ends and not just the top and bottom. In nature they can't go over the top, just around the ends and the bottom.
>Third question, why are the bottom ends of the top bar tapered upward by 1/16th inch?
So you can get a tool under them if they are glued down too much.
Most of these things are not necessary, they are just luxuries. When I make frames, I never taper the ends of the top bars.