The opening crawl of Star Wars makes it pretty explicit that the first major battle of the rebellion just happened, and it's clear from Vader's attitude that he's particularly peeved as the film opens. I don't think any of that is in any way incongruous.
As for Leia, yes I absolutely believe that Leia could talk to Vader the way she did in spite of what just happened. Because Leia has the biggest damn balls in the entire Rebellion, and that was pretty clearly established throughout the OT. This just makes that opening scene go from setting the scene to a crowning moment of badass. Consider her behavior aboard the Death Star with Tarkin. She's pretty much cool as a cucumber until Alderaan is seriously threatened and her facade breaks. She's young, feels largely indestructible, and believes that her position and family make her more or less untouchable. And she'd probably have been right under more normal circumstances, but the Death Star changes everything.
I also don't understand why anybody is confused that Leia lies, boldly, right to Vader's face. (Again, that woman has huge brass ones and I love her for it.) What is she supposed to do? "Why, yes sir. I am in fact a terrorist and a spy and by gum you've caught me!"? No, she's going to use whatever leverage she has to try and win the day, or at least delay ultimate defeat long enough for R2 to escape/find Kenobi. She knows she's been caught red handed. That was pretty clear in the original Star Wars even without RO's additions. But part of rebelling against a tyrannical galactic Empire is being bold, and being brave. I really, really don't believe that it's a coincidence that the first Rebel we meet in the films is Leia, because she's pretty much quintessential.