Here is the guide for TC and FYI beating on normal and completing a 10 question quiz getting all answers correct nets you the MGS 1 skin beating on hard and doing quiz gets you the Cyborg Ninja "Gray Fox" skin.
Mess with the best and you die like the rest. PS4- TOOHIGH2DIE420
No you can only play the one mission. But you can just run around and shoot/stealth however long you want. Finished it today and it was a pretty cool experience. Definitely worth checking out. The end quizzes are pretty tough, too. You have to get all questions right to unlock the skins.