I don't have more solid proof than the images themselves. I agree not all shots are equally grainy. I also have nothing more than my professional expertise to draw conclusions from, and my eye has spotted things in the transfer I recognize as digital post work. This is what I do. It's what Lucasfilm hires me to do for them, from time to time. You can no more make a blanket statement saying such alterations weren't made, then anyone can say they were. I didn't say they were, difinitively, I said I thought so, and still do. I have my experience to draw on; nothing more, and of course, you can completely invalidate my expertise to make your point if you want. Speculations and theories are how things start; then you go off in search of proof to refute or support the theory. This is what I've done. In this case, that proof must largely come from informed supposition and analysis, by expert eyes. I've done that, too... In any case, I hope you never get the proof you're looking for, which I think you're saying would have to come from Lucas himself. Or somebody from Lucasfilm coming right out and saying, "yes, we want to have permanently satisfied the demand for an OT while not infringing upon the sales of the remastered versions, so we made sure they looked like shit." The chances of which, I daresay, are slim.
In any case, just chalk me up as a nutjob who has a whole lot of nothing to say, and bask in the self satisfaction of your rapier perception. Then at least I will have the satisfaction of having entertained you, if I can't inform you. Six of one, really...
_Mike