Thanks, this is the sort of feedback I was hoping for when I started this thread.
My drawings on the first page are a bit outdated, but mostly correct (the second set overrides and builds on the first set). I'm using 80/20's
1530-S-Black extrusions, which are 3" wide by 1.5" deep. So the tabletop will be sunk 1.5" and the outer rail will be 3" wide on all sides. The corners will be joined with a 45° mitered edge. The top will be clamped to the outer rail from the bottom by two 1530-S cross members that are also 3" wide by 1.5" deep, along with
4365-Black plates on the sides that are 3" wide by 6" long. (See the second drawing.)
This is my biggest concern. I'm currently using 1530-S-Black extrusions for the legs, with 3" corner braces (80/20's
4336-Black 4-Hole Inside Corner Gusset). I'm not 100% sure how to calculate the load, but I'm treating it like a cantilever, where the floor is the attachment point. The leg is 27.5" long, but with the corner brace I'm treating it like a 24.5" long cantilever. With those assumptions, I get 1" of deflection at 1,011 pounds of lateral force perpendicular to the 3" side of the leg and 1" of deflection at 3,698 pounds of lateral force perpendicular to the 1.5" side.
The yield strength of 80/20's extrusions is 35,000 psi so I'm assuming I'm okay. But I don't really know. Would love a cross-check if you know better.
(The elasticity of 80/20 extrusions is 10,000,000. The moment of intertia for the 1530-S is 0.4957 on the 3" side and 1.8127 on the 1.5" side.)