In the US it would be more natural to say, “How is it going?”

In the US if you say “we’re quits”, that could mean you are breaking up with someone. Saying “we are even” really depends on the context. What kind of debt are you talking about? If it’s monetary debt, there is nothing wrong with saying we are even. Clerks use this when a transaction has been mishandled.

If someone asks you “lean in”, they are asking you to move your head closer so that you can hear what they have to say. It could be because you are in a loud place, or that someone wants to tell you something that they don’t want anyone else to hear.

I think the word knuckle head is a great word. I don't consider it to be old fashioned. It's a silly word so it can be used jokingly. Moron is a stronger word and is perfectly fine. That guy is such a moron! Unfortunately, in the US most of the strongest insults are now curse words. So go ahead and use dope, moron, idiot, imbecile , etc. Just be sure you are careful of the context and if you want that person to hear you. If you want a list of things to never say, or almost curse words you can message me. Or just watch US late showhosts