My gut feel was that he chose 70mm as an 'homage' to past 'epic' westerns, but if anything it fell flat for me after the opening credits (which gave me a late 60's/early 70's vibe, and I thought that was kind of cool), as there were no sweeping landscapes to draw attention to, so after that, if there was any kind of cinematic advantage to 70mm, I don't think I noticed.
I took your 2nd question 2 different ways; first, from a visual point of view, regarding things the characters were doing aside from the central action of the scene, and second as regarding the dynamics between the characters as they played out in front of us.
As for my first reaction, I didn't notice anything specific that foreshadowed anything significant that happened later, and the major pivotal actions not only took place off screen completely, but also QT apparently felt he needed to then circle back for us and point out what he figured the audience would have missed/couldn't have known in order to make what we already saw make more sense(?). I guess what I mean by this is it felt like QT was that overprotective parent that takes his kid for his first ever amusement park kiddie ride and just before the first thrill of whatever ride they're on, he says "okay, don't be scared, ___ is about to happen."
However, I'm guessing your second question was probably more along the lines of how the interactions/dynamic between/among the characters contributed to the story. In that case, any and all interaction between characters was kind of lessened to me by how I felt QT had set up an air of mistrust from the first scene. The first reaction from each character as they're introduced to the story and each other is suspicion. I read that tone into every interaction between every character the rest of the movie, and actually felt a little vindicated at the very end in the form of the Lincoln letter.
Also, despite the movie being so long, the action felt rushed, like no sense of time passing, which I think could have nicely ratcheted up the tension. Instead of seeing tension and paranoia between the characters grow and draw us into it, they all were already wound up tight and before I felt really invested in the story, again despite how long the movie was, the action scenes seemed to happen right on top of each other without giving things a chance to have their normal dramatic effect. Maybe QT did these things on purpose and it was part of his 'style', but there are better ways to misdirect an audience (see Fight Club or The Usual Suspects as good examples).
FWIW, I concede I had my bias against QT going into it and haven't really dug much into the background of this movie, so I'm going just by my pedestrian knowledge and experience, However, I also like to be shown things in a new light, so anything that refutes what I've said here will be given its due. TIA