I don’t think Charles would have exactly treated Anne as he treated Mary though. I mean, this is pure speculation on my part, but, taking into account how actually amyable he was to Mary, in spite of her constant whining, he would have been that much more agreeable to Anne had she chosen to marry him. As for the morning sickness hypothesis, I think Mary only had two children, they are mentioned in the chapter you have quoted from. From that we can conclude that they are both present at the time Anne came to upercross.