I have a 1/2 inch plywood with a support going width wise instead of a boxspring will the plywood break while I'm sleeping ?

It could, if you're really fat or jump on the bed. But no, it should be fine, so try it out. You won't get hurt, except intellectually. If it's not fine and cracks, squeaks and creaks every time you move.

- Then, you'll need to screw and glue a 2nd layer onto the first piece or maybe just upgrade to 3/4" plywood. Otherwise, you'd build a 2x4 frame with lots of central supports and screw the plywood to that...basically, make your own box-spring. See, I didn't say lose weight or don't jump on the bed, be who you are...just build for it.