Because it's not like the Emperor is agreeing to anything there. If Vader can set him on the dark path, you don't think Palpatine isn't thinking "ah-ha. so this kid thinks he can turn the kid and take me out. If he can turn him, I can turn him to MY ends, just like I've turned two other jedi before. No sweat."
They're sith. They don't do anything BUT hunger for power. Both Palps and Vader know this. It's matter of fact.
[quote]Plus it adds the possibility of Vader, Palpatine, and Luke co-existing as Sith in the OT... a sort of Evil trinity.[/quote]
Not really. That's never a possibility. Return of the Jedi makes that painfully clear. Palps wanted Luke to kill Vader. Palps was fully prepared to let Vader kill Luke. There would never be a possibility of a Evil trinity no matter how you tweaked the prequels.
Also, I think your switch from Dooku DEFINITELY being a sith to being some kind of "maybe-bad-guy/Dark Jedi" easily kneecaps the entire point of the character at the least, and simply makes the mystery about him redundant as well, especially considering his role in Episode III. You have to have his vagueness wrapped up in Episode II, otherwise it creates problems for Episode III. The Yellow Saber ruins that. Hell, it factors into that Return of the Jedi climax as much as anything else--Palpatine's ability to turn Dooku and get him to resign from the Jedi is a main reason he smiles and grins at Vader for suggesting he can turn Luke. you take that away from Dooku by giving him a yellow saber and not outright making him a sith, you completely weaken not only the ESB scene, but the force behind Palpatine's taunts in ROTJ.