How has the relationship between Macbeth and his wife changed since the death of Duncan?
One of the changes in the pre- and post-murder relationship between Macbeth and Lady Macbeth I find fascinating is the issue of their gender roles.
So, pre-murder, things are normal. LM gets the letter from Mac telling her of everything, and then - poof! - she doesn't want to be a woman anymore. We have the famous "unsex me" now speech of LM, who is literally asking to be rid of her empathy, consideration, etc. Generally, emotional connection is a "woman" thing, whereas the man is usually the less emotional of a couple. LM and Mac then seem to be equals until it's time to do the deed. She has a plan worked out (because Mac is too full o' the milk of human kindness and all), but when it's time to go, she can't do it all of a sudden because Duncan looks like her father sleeping. What?!? So, momentary backlash. Then, she plays woman... "Oh, Mac. You do it." He doesn't want to all of a sudden, either. He starts thinking about it, "You know, this guy is my kinsman, my cousin, my king - he's not so bad."
Then she starts yelling at Mac that she'd rather rip a nursing child from her breast and bash its head in than break a promise to Mac like he's trying to do to her (since he's talking himself out of it). So, let me get this straight? You can't kill a man who looks like your father, but you could bash an infant's skull in?
All the same, Mac has to go handle the actual murder. He comes back with daggers in hand, and all of a sudden she's back on point, yelling at him for bringing the weapons back, for blubbering, for looking like a fool. So, who's who? It's like the actual murder is the part where we see the transformation of their relationship.
Pay attention to the way they talk, too. At first, Macbeth is the one who uses the short, choppy words and sentences and LM speaks more eloquently. Then, during the murder, all of that changes, and LM's speech becomes short and choppy and Mac's is poetic.
Also, you mention the refusal to tell LM about the plan for Banquo's murder in your list up there, but I'm not sure I'm following you. Perhaps you could comment back on that with more if you need another set of eyes to look at it with you.
Same thing with the fit at the banquet -- I'm not sure that it's much more than LM trying to save her hide. It's literature, though; you can make anything you want of it with support. Seeing as Mac didn't really want to murder Duncan (seemingly so, anyway), it could be assumed that he doesn't want to kill Banquo and Fleance, either. It's really only because of the prophesies that he feels compelled to do it. So, Macbeth, who hasn't asked to be filled to the brim with cruelness, actually has some sort of conscious that feels guilty about everything on his hands. He, I think, truly flips out with guilt and remorse (womanly), while she acts like nothing's wrong and that this is something that happens all the time and doesn't have an emotional response to it but self-preservation (manly).