| if there existed this basic Sith contract that there were only ever to be two - a master and apprentice- how does a third wheel fit into the equation and why would vader and the emperor both find it a good idea, if this is contrary to established Sith order? |
I agree, Paul, that the Sith "rule of two" was a silly and unneeded prequel plot device. It's natural that evil, greedy people will try to knock each other off anyway. And "there can be only one" was taken.

It adds some more tension, perhaps, to the end of ESB by letting the audience wonder if Vader really wants Luke to join him or if he wants to kill Luke. Either of which will help to preserve Vader's life, but NOT turning him over to the Emperor as he's instructed (and does in the next film some time later.) He wasn't telling Luke at that time of ESB the Emperor would be his master, it was "join me or die".
Come ROTJ, Vader has no sense of self-preservation whatsoever, apparently. Either he really didn't care anymore as his "power-grab" moment escaped him. (If Luke chose death, over his father that's that. Let the Emperor try, (and then murder his son when he fails, or if he does seduce Luke, Vader's life is over, but it might as well be since his quest for ultimate power failed.)
However, if Vader considered himself ultimately doomed, it makes it a bit less noble that he saved Luke at the expense of his life: "Save my son, I die." "Let him join the Emperor, I die." "Don't save him, I live and call up Leia someday to see if SHE wants some power." Okay, so he chose his son's live over his lapdog life. Unless he hopes to bump into another very strong in the Force who his master doesn't sense before he does, that's what it boils down to.
Or, maybe Anakin thinks his master will bend the Sith rules. Afterall, Palpy said you shouldn't study only the narrow dogmatic vies of the Jedi, so perhaps being a kind of Force Renaissance man, he thinks Palpatine may violate the Rule of Two and not stick to the narrow, power-hungry views of the Sith. It WAS a lot easier to buy before the Rule of Two. However, Palpatine does back in 1983 ROTJ show his true colors and say "take your father's place at your side". And it's not like losing a hand was any big deal to Vader at this point. So that COULD support the rule of two, then. Or he just didn't want a father and son team working under him that might kick him out since he isn't blood.

Maybe Vader thought Palpy might stretch the rule of two since neither of them were at full strength anymore, Luke was nowhere near the strength of most fully-trained Jedi's and they needed all the help they could get.
Vader seeing Palpy as a Renaissance force user might be the best interpretation. Surprisingly, from what we've seen (with the exception of some lies not revealed until later), Palpy was pretty much always good to Vader, so he may have believed that he would have gone along with Vader bringing a third Sith into the mix. Perhaps it isn't until the Emperor orders Luke to destroy Vader that Vader sees him as a truly Sith-only in nature. This revelation helps support his decision to turn against his master again at the last minute. He might have finally become disgusted with what he was. He may never have been up to that point which could have made all the difference. "You're a hideous monster with no feelings for me whatsoever, and now you're trying to kill my good, defenseless son, the son you told me I prevented from being born!! Take this!!"
Well, I think I've done my job of making a better case for everything than Lucas ever would or could.

The most significant line change in ESB to me is the "How is that possible." 2 non-exclusive interpretations come out of that for me:
1. Vader continuing to play dumb when he's known of Skywalker since the end of ANH or shortly thereafter. (Who me? I ain't been chasin' no Skywalkers.)
2. Pretending to question his master in a confused manner, when (to those who have seen Episodes I-III, or are watching ESB again) it's actually a barb thrown at his master--"But you told me Padme (and hence, my son) was dead." How is it possible that my son survived? You lying old bastard. The fact that Vader doesn't go apeshit on his master here should have been a clue to Palpy that he was actually ahead of the curve here. Perhaps he did call Vader on his plot later on and reprimanded him for it. "Half the O2 for you today, you bad boy!."

I'm actually starting to love that there's so much open to interpretation in the Star Wars films. Even though one person can interpret things in a way that it ruins the movies or series, and another can give the stories strength. It keeps you thinking about them a lot more than if everything was spelled out for you, and doesn't that make it more fun? Otherwise it's just "I've seen it, and I can forget it now." There wouldn't be any discussions like these.
I've decided ROTS must be pretty darned good, because I've been thinking about it A LOT. One of the hardest things to buy for me was Anakin pledging loyalty to Palpy after killing Mace and getting all psycho. (post the helpless, unarmed victim act that worked really well, even if it should be obvious somebody killed all the Jedi laying in the door.) BUT, Anakin does say he'll do whatever he wants if it will save Padme. AND when Palpy whacks out and Anakin sees all the damage he did to the Jedi and realizes he played in a role in it by disarming Mace, he had to have felt TREMENDOUS fear. Fear of Palpy AND fear of what would become of him if the Jedi knew the part he played. If he hadn't joined Palpatine, he'd have told the Jedi about his part in it, and who knows what would have happened to Anakin. So, the "What have I done?!" moment resonates nicely. Had to have created a big fear-to-dark-side adrenaline rush. At that point, Anakin figures he's already F'd up big-time, so might as well make it mean something by saving his wife's life. He signs on and gets the worst assignment ever!! And what does his new master assure him: "Do not hesitate. Show no mercy. It is the only way to save your wife!" So he does the unthinkable. For as much time as the film really took to deal with everything, Lucas really did try his all to get all the "reasoning" in there. And it does make you say "No....no." rather than "Ahh, I understand" (at least after the first time it's made clear why Anakin wants more power.) Which is dramatically effective. He also made the Emperor a million times more hateful since you see how he manipulates a very vulnerable Anakin. It's like watching bullies beat up on the little half-blind kid rather than tease a strapping young boy (I'd liken the Emperor torturing Luke to that.)