Wasn't that the point in Michael being obsessed with her image in society instead of making a true love marriage with her? His father had wanted him to climb out of the Italian-American environment that you noted was regarded as déclassé by some members of the WASP establishment she represented. She was a way for him to look more polished, and for his children to “pass” more easily. Remember the scene in GFII when the smarmy Senator, who controlled whether the Corleones would get a gaming license in Vegas, said:
“I'm going to squeeze you, Corleone, because I don't like you; I don't like the kind of man you are. I despise your masquerade, and the dishonest way you pose yourself and your f*****g family.“
IIRC that was the only time the F-word was used in the Godfather movies.
I agree it was the point. And, even if another actress had played the part, I still might not have liked the portrayal of Kay. To be fair to Keaton, Coppola must have directed her to play the role the way she did. Kay came across as bland, whiny, weak, and gullible. I found a site that claims Mia Farrow, Blythe Danner, and Michelle Phillips also were considered for the part, and I'd imagine they would have portrayed Kay pretty close to the way Keaton did, as Coppola must have wanted.
No offense to Keaton. I'm just expressing an opinion here. Her tone of voice and mannerisms are more suitable in other movies.
It's fun to think of other actors and actresses in these roles. At the moment, I cannot think of another actress in the 1970's who might have fit the role of Kay, but I can think of a few actresses who fit the picture of an upper-crust WASP. Sigourney Weaver is one example, although she would not have been suitable for the part of the vulnerable Kay, either. But, an actress like Robin Wright or Suzy Amis might have worked. They were still children when The Godfather was filmed, but an actress with their tone of voice and temperament would have served as a stark contrast to Pacino's portrayal of Michael.
As a side note, many people on this forum talk about "white culture," and they want the word "white" to be capitalized. The Godfather illustrates there is no such thing. There are big differences among European cultures.