Samsung has currently not released the update for all S7s. It's a slow rollout, keep checking!
Ahhh wait, here's a problem: http://www.phonearena.com/news/Samsung-Galaxy-S7-edge-wont-receive-Android-7.0-Nougat-in-all-regions-in-Q1-2017_id90722
My Lenovo vibe x2 also has the ram problem, same 2gb ram, only around 600-700 MB available even without any apps on.
About storage, the 6GB ur missing goes in all the default apps and the Android software.
Yes, putting an M919 screen on the M919V will work. You can go through these steps for help: https://www.ifixit.com/Guide/Samsung+Galaxy+S4+Cracked+Front+Glass+Replacement/21843.
As you can see, iFixit has rated it Difficult, so you might have to do it extra carefully.
Note to Moderator...