There are several things I didn't understand about the series: Please read below.

- Because he just did. The diary needed to be destroyed, and he had a weapon in his hands.
- I think you mean Goblet of Fire. And he tried to tell Fudge, but he wouldn't listen. I think it was implied that Harry told Dumbledore everything. As a sidenote, expelling Malfoy would be a stupid thing to do - who was to say if Draco would become a Death Eater just because his father was? Not only would that be un-called for and the Ministry wouldn't approve because no one believed Voldemort was back, but Ron and Hermione argued with Harry all throughout Half-Blood Prince about how unlikely it would be - and it would be completely out of Dumbledore's character to expel someone; he trusts people where others don't. Severus Snape is a prime example of this trust.
- Harry, Ron, and Hermione already knew all the spells they needed. What he purposely didn't tell them about was the Deathly Hallows, and he did this because he didn't want Harry to rush into things for the power and make the same mistakes he did, and he knew Hermione's doubt and more closed mind would prevent him for rushing into things when they discovered them.
- I'm assuming you were referring to the actors, not the characters? Emma Thompson plays Trelawney, doesn't she? Anyway, Trelawney was incredibly important to the plot. She was the one who made the prediction about the whole "Neither can live while the other survives" and Harry being the "chosen one" and all, and Snape was the one who overheard and tipped off the prediction to Voldemort. If she wasn't in the story... well, the story wouldn't exist. Sorry, I can't remember who Kenneth Branaugh plays.

I hate to be blunt, but I'm just going to ask: Have you read the books? I know this is going to make me sound arrogant if you really have, but a lot of my points were explained in the books, and anyone who paid attention while reading them would know most of these answers.

1) Tom Riddle's memory was kept alive inside the diary. Harry didn't know either that the Basilisk's venom destroys Horcruxes or even what a Horcrux is. It was just a simple reaction, an instictive effort to destroy Riddle.
2)Harry told Dumbledore everything about the graveyard. But it was in vain because, if you remember, Dumbledore was considered a mad old man and Harry a psycho with a pathological need for attention by the Ministry and the magical society!So who would listen to them blaming Malfoy, who belonged to the elite? Besides, I don't think Dumbledore needed Harry's confirmation to believe that Malfoy was still faithful to Voldemort.
3)As far as they could, they always used spells to protect themselves. Whether someone died, it's simply because they didn't react too fast or something like that. But you have made a mistake, there is no spell to turn something dangerous into something ridiculus. This applies only to Boggarts, which are creatures that have the ability to turn into something fearsome. You can use Riddikulus to a Boggart-Dementor, but not to a real Dementor. I must also add that the real creature is always far more dangerous than the imposter(boggart).
4)Throughout the Harry Potter series there must be hundreds of characters, that serve Rowling's will to make the magical world seem as realistic as it can get. It's literally a whole world, so you can't expect all of them to be vital for the plot. Yet, Trelawney (Thompson) is a very important character. She made the prophecy about Harry and Voldemort, so she is the beginning of the story! As for Lochhart(Branaugh), it's true that he is no key to the plot at all. He still is a message though: he shows where idiocy along with boasting can lead you!
I hope I helped a little!

-He wanted to destroy it and the fang was the only thing he was holding. He didn't know what would happen but it was his last option.
-It's Goblet of Fire, and just because we didn't see Harry tell Dumbledore doesn't mean he didn't tell him. Besides, as Fudge was twisted around Malfoy's finger it wouldn't have made such a difference.
-It depends what he didn't say - some things he forgot., and others he refused to tell because he thought Dumbledore wouldn't be interested (in Order of the Phoenix when Dumbledore was avoiding him).
-They are kids. Inexperienced teenagers who were thrown into situations that were too advanced for them and their maturity level. They were afraid, they panicked and they forgot.
-That only works on a boggart.
-Do you mean having the characters or having those particular actors? The characters Lockhart and Trelawney were very important to the plot - Lockhart was used to show the incompetence of authority and that not everything that looks and sounds pretty is good and honest. Trelawney is arguably one of the most plot-important people in the whole series - without her, there would have been no prophecy - Lily and James would have no been hunted down, Harry wouldn't have his scar and destiny, Snape would still be a Death Eater, Voldemort would likely still be active... As for the actors - they are great actors, which is quite nice to have in a film series.

1. it was instinct. I mean what would do if a book was draining the life out of your best friend sister and bringing your enemy back to life and you had in your hand a very dangerous and deadly weapon?
2.first of all that was in the goblet of fire, he was already removed from the board of governess in book three, and why would you expel Draco Malfoy for doing something that he didn't do but his father did? but to answer your question he either thought that Dumbledore already knew and he couldn't do anything about it or the fact that he did tell them in the hospital wing when he heard them arguing about the dementors taking barty crouch jr. soul and he started to say the names of the death eaters and the minister ignored him so Dumbledore didn't have enough power to do anything about it.
3. I don't really don't know why the only used those couple of spells when he went through six years of Hogwarts school.
4. they had them there because if they didn't have a full staff then it would appear weird so they had to have a muggle studies professor and a defense against the dark arts professor since snape was promoted to headmaster he couldn't teach as well.

Ok slow down. Harry had no idea. He just played it by year.
He did not tell Dumbledore because he wanted the Malfoys to die. The ministry did not know what they were doing at the time.
Harry did not tell Dumbldore because he did not want to disturb a mind already like that. The students in the end fought til their last breathe and also they can't have RIDICULOUS unless on a boggart. Also, i am not the directors so i have no idea why they chose them to act as a guest teacher.