Actually, the only reason he needed this whole 'retaining one's identity in the Force' BS is because he killed off Obi-wan in ANH, and then wanted to bring him back in ESB.
"Prior to the PT, all Jedi were SUPPOSED to disappear when they died, Lucas just stuffed it up. No BS explanations required. Case closed."
Actually, prior to the PT, Lucas rewrote the events on ROTJ, stating that Anakin's body eventually disappeared from the armor, and Luke was dragging the empty armor on board.
"This would indicate that A) Not every Jedi knew the 'trick' yet"
The fact that we only see Obi-wan's ghost in ANH should be proof of that, not to mention Vader's insistence in ESB that Obi-wan could no longer help him.
"We shouldn't have to turn to books and other resources to find out stuff that we should've learned in the movie itself."
Agreed as well.