I bit the bullet, took yesterday off early and picked up a couple straight 2×4s and immediately set about to cutting them to length. I disassembled the warped frame and cut one of the outer rims down into the new framing members since the previous ones were not exactly cut square and the screw holes might be a problem.
My circular saw skills are certainly improving. I managed to cut every piece exactly to length and dead square which is quite the accomplishment for me. I used the wider shelf plywood to assemble half of the frame using clamps along each point of fastening. Then I switched to the mating shelf plywood for this assembly since it is the same width and simply left half the frame hanging off the saw horse and then moved the plywood to the other side once I was halfway through.
The plywood for the shelf is in 2 pieces which is why I had forgone using it the 1st go around for clamping. Once I had this frame assembled it was perfectly flush, square and plumb. I would have gone ahead and fastened the shelving plywood to it but it was time for dinner and at least 1 hour of my work time is spent lugging materials and tools up and down the basement steps so I can work on my carport.
I expect the shelving material to go on without issue but I also expect the legs to be a major problem due to the unevenness of my floor. I am loathe to cut the legs to fit the floor in case I decide to relocate the workbench elsewhere in the basement or to another place altogether. One thing at a time I suppose, but the legs are the next step in the process. For now I am considering at least placing a level on a board and lifting it it up from where the outside legs will be to get a clear idea of just how much longer the front legs will need to be from the back legs.