You give very good explanations here.

I kind of figured that "This cannot hold" also could mean that he believes as she matures she will see that "no second attachments" is not very rational. And your explanation that he didn't believe it himself till recently may explain some of his gravity. Very enlightening. (It's possible too that their are other reasons too for his "oppression of spirits")

"But a total a change of sentiments, no, no do not desire it"
He would hate for her to lose her idealism, naivety, innocence. Yes that does seem to be what he admires in her.

Of the "opinions too common, too dangerous"- He seems to clearly referencing the unfortunate lady from an "enforced change". Perhaps lost all her idealism and perhaps any respect for herself. Maybe the cynicism that came from her new situation caused her to have a new outlook on the world and behave differently because of it. I can't find the right words so hopefully this makes some sense.