Bakala

You don't have the latest. When you start the game it says the version in the bottom right corner.
 
No problems adding it manually to Nvidia controlpanel
 
As a non steam game, yes. Otherwise no.
 
Hard drives are much slower than SSDs and will cause longer loading times and possibly texture pop-in
 
I would use Geforce Experience to get baseline settings, then change them how you like.
 
Edit~ you can see your game version in multiple locations: 
 
http://i.imgur.com/gvYbhZw.png The first string of numbers (393.2) is the game's build version, the second (1.0.6.1) Is the GTAV Launcher version
http://i.imgur.com/DQpdiIj.png The first number (393.2) is the game's build version. The second number (1.28) Is the GTA Online version.
http://i.imgur.com/eigqWtO.png You can only get to this setting by being in GTA Online only!

1. There should something in the pause menu that says the game version. It will install the version on the disk. It should then update to the latest version when you launch it.
2. Shouldn't be any problems
3. You can add a shortcut in Steam that launches the game, but you won't get the achievements, screenshot library and other things that you get with Steam games. There's a button in the library section of Steam. It should say something like "Add a game to library"
4. Hard Drives influence load times, not in game performance. 
 
EDIT: Here's a video for adding the game to Steam
 

Start -> Search this: Desktop Icon Settings
 
Un-check the Network icon box.
If it's already unchecked, just check, save, uncheck and it'll go away. (this happens once in a while)
 
 
That's it.