Well first off, just want to say that discussions about the answers to all three of these questions have been held on numerous times elsewhere, so if you're interested in various points of view, you might want to wade through the forums in hopes of finding them. That being said, I'll toss in my two cents.
1) In other discussions, I've argued that the reason why Obiwan and Yodi disappeared while Vader and Quigon didn't depends on the preparedness of the individual to let go of their life. Obiwan choose to let Vader kill him, Yodi felt it was his time to die. Both of them were prepare to leave their life behind, and as such were ability to free their spirit from there bodies and join with the Force.
Quigon wasn't ready to die... we can see the shock on his face at the saber hits him. He's wants to stay alive to guide Anakin, as he feels he's meant to do. True, he has Obiwan take over for him, but he wasn't ready to go. Vader also doesn't appear to be ready to go. He realizes he's dying, noting that nothing can prevent it, but he seems rather resigned in the fact. He realizes he's been saved, and to that extent he's at peace, but he doesn't seem actually ready to die. Basically since Vader and Quigon are too tied to the physical world, they need help freeing their spirits from their body, which is accomplished through the pyre. The body is destroyed, allowing the spirit to leave and join with the Force.
2) This has been covered pretty well by the other responders. The only thing I might add on top of it is that many people look to Yoda for guidance, through all stages of their jedihood. Obiwan could very well be talking about seeking Yoda's help with numerous things after he took Anakin as a Padawan. After all, he refers to Yoda as "The Jedi Master who instructed me" not as "my Master." It could simply be that Yoda was a master who instructed Obiwan to undertake various missions and provided him with counsel, not necessary the person who taught him as a teacher would a student.
3) While the argument of Boba's life after the Sarlaac was presented in EU (none of which I've read), some of the support for it as being possible canon arises from the fact that the third trilogy was never made. We KNOW that Lucas originally intended for there to be a third trilogy. So it's possible that Fett might have reappeared within that trilogy, meaning his escape from the Sarlaac would become canon (though not necessary as described in the EU). Since Lucas isn't going to make the third trilogy, then canonically, Fett is dead. Still, who knows, maybe 10 years down the line Lucas will change his mind and make the third trilogy (or set someone else up to do it) and canon will change...