Wow lot of questions.
1- The mark on Neji's forehead is the Seal the Hyuga Main Family puts on the members of the Branch Family. This Seal allows the Main Family to torture or kill the Branch Family member.
2- He actually healed Hinata, but he beat her after it. Kabuto was just beginning his job, which was making everyone there unconcious. He took advantage of the situation and already made Hinata and Kiba unconcious.
3- Yes.
4- They knew each other in the academy. All Leaf Genins knew each other, and almost all of them are very friends.
5- He was just there to say he'd kill Sasuke.
6- Gaara has a kind of monster inside his body. It's Shukkaku, an old Sand priest sealed within a tea kettle.
It's not a demon.
7- Sorta. He wants to become the strongest ninja ever, kill the Kages and rule the world. For this he needs an Immortal lifer and the Sharingan- since he considers strenght the number of jutsu you know, the only way he can become the strongest is to know all. For that, he needs the Sharingan.
8- Female Oro? :P You mean the Grass nin or Anko? The Grass nin was just Oro disguised. Anko was his student, that's why she has snake-based jutsu, like her sensei.
9- No clues. There are theories, also. That were lost within the forum crash... but the 4th is related to Naruto just by the fact he sealed Kyuubi within him. If it's blood related, we don't know.
10- No, that was Shukkaku of the Sands. See answer #6 to know who's Shukkaku.
11- In the manga, now he can fight Naruto equally, he may be even able to beat him. I'm talking about chapter 191. Where you stopped, chapter 153, Sasuke doesn't have a chance against Naruto.
12- No, Itachi isn't working for Orochimaru. Itachi is working to the Akatsuki, the world most famous and feared criminal organization formed by the best nins. His duty is to kidnap Naruto to steal the Kyuubi's power, not kill him.
13- Jiraiya took Naruto with him to train him, and also get a company to search for Tsunade. Who's Tsunade? The last of the Sannin. She was along with Jiraiya and Oro a student of the 3rd Hokage.
14- He may have had a reason. Back then I made some theories about it. But he hasn't mentioned yet. The only thing we know about the Uchiha's clan destruction is that apparently Itachi killed all the Uchihas besides Sasuke.
15- I prefer the true meaning of sensei to Jiraiya. He's Naruto's true master. A father figure for Naruto would be Iruka-sensei.
16- Itachi used genjutsu on the woman to distract Jiraiya. He, of course, as a pervert, went after the woman, giving free way to Itachi and Hoshigaki Kizame to kidnap Naruto. Too good Jiraiya got there on time to save Naruto.
EDITING: Beat by
Moon-san. Well, my answers are a bit more complete
Just an additional help from the Naruto-sempai.