In Part III while in Sicily, Michael & Kay have a good, long talk and "clear the air" about their past. Michael explains himself to Kay and the reasons for his actions that we see in GFII. While I don't believe that they would have gotten back together as a couple, I do believe they were both doing a lot toward mending their relationship. Then Calo comes in to deliver the news of Tommasino's murder to Michael. Kay witnesses the exchange and knows that Michael is okaying Calo to avenge Tommasino's murderer. She comments, "It never ends."

Even worse, I would venture to say that Kay most likely blamed Mary's death on Michael, at least to some degree.

My question is had these two events not occurred, do you think Michael & Kay would have had a civil relationship again? I doubt that Kay would have left her 2nd husband, but it was obvious that she still had strong feelings for Michael. We are left to assume that Michael probably rarely, if ever, heard from Kay again since the time of Mary's death to Michael's death at the end of the film.