That's all probably true, but, in an alternate timeline where Robbins gets Moreno to audition (assuming that this story she told was true) and she is cast by the creative team (including Bernstein), I doubt that they would then have wanted her to try to be a soprano and sing the role as we currently know it. That would have been weird. So I have to think that if they'd wanted Moreno for the role, it's the role that would have changed, since it's unlikely that the actress could change in that way.
For all those reasons, I agree that it's doubtful that Bernstein would have been on board with her casting, no matter how great her audition might have been for the other aspects of the role. But, if he had been, I think my original point is valid...that the role of Maria would be very different from what we now know.