Actually, it was an actual body double for Tarkin in Sith, not CG.
As for the others, yeah, it's crap. When watching the trilogy, I always considered Yoda and Palpatine avatars of their own sides of the force. They weren't knights like Luke, Ben, and Vader. Palpatine and Yoda used their minds, and the force lightning was a trait that showed that Palpatine was a bigger badass than anyone seen thus far. So, no, I never thought about Vader having or not having the capacity to lightning fry someone. But then the prequels came along and gave both Yoda and Palpatine lightsabers, so... yeah.... But to answer your question more directly, I never even heard before that there was a reason why Vader couldn't use lightning or that it had even come up. But I guess it is a question that would come up with Dooku having used it. Since Vader was supposed to be better than Dooku, it suddenly becomes relevant. I'm sure it was just made up to explain the continuity flaw.
Next... the lightsaber fights. Not only do I find the whole "old men vs. robots" argument to be utter rubbish and ridiculous-sounding... it exists to justify the fights in the prequels, but it is contradicted by the fights in the prequels, as we have old men (Dooku) and robots (Grievous) fighting in the prequel style. That should be evidence enough to dispel the thought that that idea was actually in George's mind while filming the originals.
As for Vader's physical state, again, I never thought of it before George made that claim. And if it's true, then it greatly diminishes what Vader is supposed to be. Even in the first film, where he's just the "dragon" rather than an overarching villain he is still shown to be an incredible and fearsome warrior not to mention a formidable pilot. He's menacing and powerful. While in ROTJ, it's an ironic shock that underneath is a broken old man, if he were to be physically broken-down and pathetic, then he wouldn't be Darth Vader. Furthermore, it always confused me in terms of the prequels. The whole time Palpatine is trying to upgrade his apprentices with Anakin being the ultimate in that. If Vader is so horribly debilitated in his suit and his midichlorian count so diminished with his missing limbs, then you'd think Palpatine would just dump him as a "necessary loss" and grab up the next best force-user he could manipulate. Obviously Vader has to have a lot going for him.
And, finally, as for Tarkin and Vader, that's simply explained. In the context of the first movie, the Sith were a warrior tribe who allied themselves with the Empire. The Empire itself was not ruled by a Sith. The Emperor was a politician, and Vader, a Sith, was working for the Empire. The Empire obviously respected (to a certain degree) the powers of the Sith, but, again, Vader was considered a "dragon." The true Empire was built on military strength, not the force. Obviously that gets a bit more difficult to swallow when the Emperor is retconned to be a Sith.