Can't really see the mark indicating where you want the trap nor have a good idea of the shape and size of the shower you want to build. There will be no issue with having the inlet and outlet of the trap rotated (folded) as long as you don't throw additional elbows into the equation. For maximum strength, you want any patches in your ply to span at least three joists. That way, the middle (at least) of the ply is supported, and it is not hanging just by the edges and the fasteners on 1/2 of a joist width. This can be an issue when you want cement and tile on top of the surface.
On a conventional shower, it's nicest to have the drain as close to the middle of the shower as possible. On an ADA, curbless shower (is that what you're talking about doing), it's nice to do something like a linear drain against say the back wall of the shower.