Workbench cabinet - structural question - P screw placement

[Laura]

Hello, I got a LAB workbench for free. It is a very expensive one with fantastic wheels and I want to use it. However I would like to convert it into a workbench. I would like to create the cabinet that seats on the two side arms. The workbench is 60"x18" and the cabinet will be 60x15x12. I am building with 5/8" plywood but I can do sides with 3/4" ply if necessary. Considering how long it is I was planning to have the back made of 5/8" plywood instead of just a couple of cleats. The questions are:

a) Should the sides and back (5/8" too) overlap the bottom? This means that the pocket holes will be in the bottom piece of the cab

b) does it matter  (structurally) if the holes are inside or outside the cab?

c) the two sides will overlap the back and the bottom, this means that the back will have pocket holes to attach to the sides. Would it be enough to prevent sagging?

d) Do I really need a face frame to keep the bottom from sagging?

e) Would the partitions (at least 2 made of 3/4 ply) be enough to keep bottom and top from sagging?

I have attached pics:

1) the table I have

2) the drawers I would built

3) My dream workbench (all wood though)