When Leia says "Chewie, just do it" she is facing away from the camera, so it is hard to tell for sure, but after having heard the alternate line, the more strongly repeated "We have to go back!", I noticed that the movement of her mouth doesn't seem to quite match up with the line as spoken in the 35--it probably matches the other, because that could have been what Carrie Fisher actually said when they were filming. So I'll grant that one is probably authentic. ;)

And I'm not saying that these alternate lines just sprang up from nowhere--several of them probably were in the 70mm version. But Starmike, who claimed to have an ESB 70mm recording, said that R2 didn't taste very good in that version also, and if that's true, it would mean that Luke saying he was lucky to get out of there in the SE doesn't represent any previous mix. Both lines would have been recorded, but just the one may have been used in both versions originally--and then for whatever reason they went back to the line as written for the SE, rather that the funnier improvisation. So just because a different take is used in one version, it doesn't mean that it also appeared in another, and there's no good way of telling which is which, unfortunately.

I do like the dialogue as used the 35mm mix better--the alternate ones are sometimes good but usually seem strange and wrong somehow. I have the same reaction when hearing the mono mix of Star Wars. So the 70 wouldn't be my preferred listening choice in any case. I'm rather grateful for the 1993 version actually--the mixing choices of the 35 but with far superior dynamic range. But a 70 would be worthwhile, if the authenticity issue could be sorted out with greater certainty.