if you fix the hinge so that the knuckle doesn't touch either the gate or the post you should be fine. It just stops the hinge from binding on the gate or the post.
Mafell is a german tool company and they do a circula saw that when you lay it on it's side will cut 100 mm up from the floo level. It's specifically designed for this purpose and is a tool that is simply amazing to use.
The internal angles should be secret mitred. As for the 120 angles what to do here is to take the skirting and lay it flat on the floor up against the wall and draw a line along the edge of the skirting. Do the same on the opposit side if the joint. Then take a sliding bevel and line up the...
The size of frame fixing is relative to the weight of the gate. Overkill is better than chancing it and the gate hanging off the next day. If its a standard ledged and braced gate then you just put the hinges in the middle of the crossbars. For a gate you should look for about 8 - 10mm clearance...
Plasterboard is plenty strong enough for the job, but plasterboard is not waterproof and when water spills over the sid of the bath then it will become soggy and will fall apart. Also using plasterboard means that access is very difficult under the bath.
I really wouldn't worry about your WBP ply as it's an exterior grade ply which means that it's waterproof. As for delamination due to moisture this simply won't happen as the glue used is Water and Boil Proof. Im my experience this ply hasn't delaminated and i use it for soffits and boxes and...