fortymile,
Modern scopes are indeed built so they can't be disassembled too far without special tools. However, the nitrogen will generally leak out very slowly anyway. It's used inside the scope only to make sure the interior is dry after assembly.
Parallax occurs when the reticle isn't focused for that range--the reason you should try both to adjust-out parallax and focus the reticle, using whatever means the scope includes.
