1) Yes. A mod confirmed that the answers are case sensitive in this post: https://forum-narutoen.oasgames.com/page/show-post-39228-1.html

2) I believe the base ninja's first name is the only thing you should enter. I.e., if you thought it was Sage Mode Naruto, you would just enter Naruto. There was another post like the one above that got a more in-depth answer, but it has since been deleted, probably because people were spoiling the upcoming mystery ninjas.

3) No, the game doesn't tell you if your guess is correct or incorrect. You can check what guesses you've made by going to the guess page again. You'll only find out whether you've guessed correctly or not when the mystery ninja is revealed (although it's been spoiled pretty heavily in these forums).

P.S. It's not Tenten.