Betty really gave Don a second chance to pull himself up.  She genuinely appreciated Don taking care of the baby in the middle of the night, then all of a sudden JFK died.  While most of the nation was transfixed by the event, putting everything aside to take in what was going on, Don strolls in and says "why are the kids watching this."  Clearly, the events in Dallas were a chance for everyone to reveal the depths of their own humanity.  Don was completely unfazed.  The final straw was showing no reaction to the shooting of Oswald, despite the shriek and terror of his wife.  No humanity?  Sayonara Don, it's been a slice.

Just offered that theory to my wife who totally disagrees.  She thinks that it's just easier for her to move on now that Henry's back in the picture.  I still like my theory.

I'm not sure I understand the connections you're trying to make here. 

If anyone seemed unaffected by what transpired, it was the kids! 

Anyway, if there was anything telling about Don's questioning why Sally and Bobby were watching coverage of the mayhem, it's that his first reaction was to pull the veil over their eyes rather than present them with the truth.  Maybe Betty was disgusted to see that Don's instinctual reaction is to avoid reality rather than confront it?  This would tie in pretty well with what Betty said to Henry when they parked shortly after; how she "didn't care" what Don might think in her absence, since he's been lying to her "for years."

Also, keep in mind that Don Draper is always "in character," so to speak.  On a number of levels.  The image he projects is that he is cool and collected at all times, which is obviously not the case.  We saw Betty break through his stone exterior and expose the soft and gooey core first when she got his teary confession out of him, and more recently, when she told him in plain English that she didn't love him anymore.  His autopilot instructed him to say that she didn't know what she was saying, but in private, he collapsed in that bedroom chair almost like he had been shot. 

Man, I've got to learn how to condense.