Context is what matters. Marriage was a Type of Israel's relationship with God. Jesus said that if a man sends his wife away, he causes her to commit adultery. At face value, He was speaking of marriage. But at the heart of the matter he was saying that Israel had left her espoused God and traded this God for another, committing spiritual adultery. Israel, God's rightful people, had pledged herself to an alternate god. This was Jesus' critique. Paul carries this Typology into every human institution in Ephesians, including slavery. What is important is to know that God will never send His Bride away and to only enter into human institutions where we can make that same type of committment.
Yes, but it's only if you divorce the person for the wrong reasons. Example, if you divorced your wife because you thought some girl in a bikini was hot, that would be wrong. But if you divorce someone for the right reasons and find someone who really cares about you, when they ask for marriage what's wrong with that? You can't just say "Well according to the Bible I'm still married to the person I divorced for some reason and I can't marry you or I'll die and go to hell". Seriously.