Except for the sniper knee, return edges are not required. Return edge gives the armor the illusion of thickness that's all, remove as necessary. Also, if it comes done to it you can even cut out the back as seen here in this reference photo.
Regarding the back (opening), I would only remove the flare in the plastic and leave the rest. The muscles in that portion of your leg tend to flex the most so trust me when I say you'll want room to move around in. You can always add fabric foam to stabilize the interior if needed.