]p.s. Why doesn't Henry seem to care about his brother's behaviour to Isabella? Is it possible that he knows enough about Isabella to know that James would be much better off not marrying her, and that his brother's attentions to her would break up the engagement without any serious consequences to Frederick?

I would say yes. When Henry first sees Isabella, she dances with his brother though Catherine says that IT had no intinion of dancing. Henry has been in the world enough to know a flirtation when he sees it (though it's not like IT and CT tried to hide it). He no doubt realized that IT would bring nothing but woe to the Moreland family and knew that CT would never be in any danger of her.