This is some pretty funny stuff (sorry, but the Yoda thing was pretty funny). I would probably stick to what is in the actual filmed STAR WARS stuff and do the overdubbing, which seemed to work the best (the actual ROBOT CHICKEN part with Palps waiting for his luggage kinda threw everything out of whack, as we just saw him go from human to clay).

Maybe you could play with have Vader's voice modulator "malfunction", which would explain his voice alternating from James Earl to Stewie (some of the lines where Stewie says the same exact line as JEJ have it cut back and forth to get the idea across - maybe show a shot of Vader messing with his chest buttons to try to fix it).

I WOULD shy away from all the "super-sped up stuff" (as it comes across as forced to me), unless of course you are planning on adding Benny Hill music.