Of all the unfortunate things in this case, Marina's obvious intense dislike of Ruth Paine is truly surprising. When you read Marina's testimony and comments about Ruth Paine in the aftermath it is perplexing that she clearly disliked Ruth intensely after the assassination. Ruth Paine was obviously trying to do a good deed in assisting Marina and her family when she had no obligation to do so. My guess is that Marina was concerned that Ruth Paine could reveal some details about her that would not be complimentary. Likely influenced by Robert Oswald who was suspicious of Ruth. Marina also wanted the limelight and didn't want to share it with anyone. There were likely financial incentives to be the source of information about Oswald and Ruth also knew some of those details.
Marina had plenty of cause to be concerned. She knew that her husband had attempted to assassinate a public figure in Dallas. She knew that her husband kept the rifle in the Paine's garage. She knew that her husband made an unexpected trip to the house where he kept the rifle on the very night before the arrival of the president. She knew that her husband worked in a building overlooking the motorcade route. Should she have been concerned being the only person on Earth to know these details on Nov. 21 and perhaps have been in a position to stop what happened? Maybe. She has gotten off easy in my opinion. Maybe it is unreasonable to suggest that she should have known that her husband was a threat to assassinate JFK but she had plenty of cause to give it some consideration.
On the split between Ruth and Marina: As you pointed out, I think that came out, to a large extent, due to the influence by Robert over her after the assassination. He detested the Paines, especially Mike, and I think he convinced Marina that they were using the assassination and her for their own personal gain.
I'll go to McMillan again. On the breakup between Marina and Ruth:
And on Marina's guilt: