Because Adam is not Hannah's friend at this point, and Jessa is (was). Just like if your spouse cheat on you with a random person, it's your spouse that's more at fault (and the other person is at fault by enabling your spouse), because it's your spouse who made a vow to you.

Because that girlfriend is someone who continuously went out of her way to help you? And when Jessa choose Adam over Hannah, she and Adam have no such relationship? It's not about choosing a girlfriend or a man, it's about trying not hurt a person who have been a pretty damn decent friend to you (while you have not).

They weren't exactly a "race". They were a cultural group. And certain cultures have certain values.

For the purpose of taking back her earldom? Which seems to be the only semi-reliable way to survive (and to protect her son/her people/women and children who look up to her).