As for grammerical errors, after reading that post I checked the synopsis my self. Here's what I found outside of what was reported.
ANH
It should be "possession" not "possesion"
Star Destroyer should be capitalized.
I've also been told to never start a sentence with "but" nor "and", but that's debatable.
ESB
Empire's pretty solid, all I found were some nitpicks.
"The rebels' only hope is to hold off the Imperial troops long enough, so the Allience transport ships can find their way to safety through the Imperial blockade."
That sentence, while grammerically correct, is much more ungainly then need be. Might I suggest the following.
"The rebels' only hope is to hold off the Imperial troops long enough to allow the Allience transport ships to find their way safely past the Imperial blockade."
or
"The rebels' only hope is to hold off the Imperial troops long enough while the Alliance transport ships find their way safely pass the Imperial blockade."
In any case it's a mouthful
"finds out that his friends"
works either way.
RotJ
The first paragraph is indeed a run-on sentence. One I have no clue as to fixing at the moment.
Spell out "second"
"Before the Rebel fleet can attack the station, a small group of soldiers, led by General Solo, will have to take out the station's shield generator, located on the forest moon."
I'm a bit unsure on this. It's possible the commas before and after "led by General Solo" and right before "located" aren't necessary.
"and Vader's master" instead of "and his master".
These are all suggestions on an otherwise great piece of work.