So much weaker? Prove up. Not much time passed between the movies.
Your theory holds no water, especially since Luke wouldn't have done Vader much good in overthrowing the Emperor if he didn't have a hand.
There is that little matter of Luke having over ten times as much training between ESB and ROTJ then the rest of his life in ANH. I guess you just missed that minor detail.
Yes, if Vader were actually trying to defeat/kill him. He wasn't, though.
The same thing happened with Luke and Vader in ROTJ. Luke didn't want to kill Vader, Vader went to far and Luke went all out just as Vader did in on Luke after Luke clipped his shoulder in ESB.