It should be .71875" thick, commonly called nominal 3/4". So look at the stamping on the wood or for a label one one side, it sounds like a piece of 15/32 (0.46875") got put in the wrong pile.
For full size bed (74 x 54), you can do it with 1 sheet of plywood if you use some supporting wood. I used 2x4s to form the rectangle perimeter and 3 inside the 74x54 perimeter frame. you then have your sheet of plywood cut so you have a piece 48x54; from the remaining 42" length you cut a 26" piece and then from the 42x22 now remaining you cut a 12x26. Screw pieces to the frame/support you created. You'll want to position the 3 inside supports so that you make sure you have good support position for the 12x26 piece.
If you can get it cut at a store you would be at the mercy of how square the saw is and how accurate the crew is.
Highly unlikely unless it's laying down on a floor; even then it probably wouldn't be the best choice
22/32 would be 3/4" yes.
No, I don't think it would be good for outdoor use. Unless you paint it, maybe. But not just untreated
Shelving is what I used it for.
Yes. I got the one I wanted and the lumber associate cut it into three pieces and at the size I asked for.
NO! Not suitable for any cabinets at all IMHO. Full of voids between layers.