We had quite a few discussions years ago about Kay going along with Michael for all those years, knowing all the things you cited. Many posters believed that Kay thought she could "change" Michael, although there's no evidence of her trying to do that in the movies (Michael says in the Washington hotel scene that he "knows he can change").

Kay's "horror" began at the very end of GF, when Kay sees Clem kiss Michael's hand and says, formally, "Don Corleone"--see that horrified look on her face when she recognizes that whatever rationalizations she may have made about Michael's mysterious business gave way to the realization that he was, indeed, a Mafia Don. People can deceive themselves into believing what they want to believe--but not forever. My guess is that Kay thereafter behaved like a typical well-bred wife of the era: "love" gradually giving way to
"duty" toward the wifely responsibilities to preserve the marriage and keep a two-parent upbringing for her kids. Wives of that era were conditioned to believe that those burdens fell mostly on her, not on him; and that divorce represented a failure of hers, not his. I grew up in that era, and I can tell you that divorced women (but not men) were shunned, as if they had the proverbial Scarlet Letter on their foreheads. But the Tahoe shooting--the direct threat to her, her children and her home--pushed her over the edge.