Skyjedi, a blowup to 70mm wouldn't increase grain, as 70mm negative has a very fine grain structure. That's why the effects for Blade Runner and Close Encounters of the Third Kind were done with 70mm - so the grain wasn't excessive when compositing multiple layers of film.

I also doubt it would exacerbate the garbage matte issue, assuming the color timing was done correctly.

As long as the blowup was done from the original IP, a scan from the 70mm master negative would probably be the next best thing to scanning the original negative if that (somehow) wasn't an option (well, the original IP would be the next best thing, but, you know, barring that ... )