Great post!
I believe that Castro and Cuba were his main motives. LHO and Marina reportedly fought like cats and dogs quite regularly. So I doubt that her refusal to immediately move back in with him had much bearing on his decision. (Other than perhaps that he wanted to make her feel guilty about it.)
Charles: Like you, I don't believe Oswald suddenly became "apolitical" on the day of the assassination. He was a political person throughout his adult life and he was deeply estranged from America. He repeatedly denounced the US economic and political systems (sometimes correctly such as segregation) and whether he was a "real" Marxist (as he understood the term) he certainly didn't like America.
However, we have the odd lack of planning for the assassination of JFK. In contrast to his planning for Walker. He retrieves his rifle the day before the assassination - he doesn't get it earlier to practice with it. He had four bullets. He gets a rid from Frazier. He keeps $15.
All of this is so sudden, so last second, so spur of the moment. If he had deeper reasons for killing JFK - such as Cuba and Castro (as I think he did) - then why act so last moment? There's little planning involved.
And to be fair to the Oswald defenders and their argument: He got damned lucky. He was able to be alone at just the right time, to have JFK pass right before him. It does "look" like JFK was brought to him. Of course, this also makes any framing of Oswald difficult to believe. How would the framers know where Oswald was at the time of the shooting? How could they know he didn't have an alibi? In order to frame him he can't have an alibi.
The evidence for me is strong that he shot JFK. And the evidence for an alternate explanation is so weak and the explanation so complicated and convoluted as to belie logic. This was faked and that was faked and this was planted and that was planted. Good lord, it's absurd; layer upon layer upon layer of fakery.