Her: Oh my god you're so posh!
Me: I know, and you're so common, you love it!
She laughed, because she was already attracted and we were vibing like this for a while, just teasing back and forth. She wasn't really common and I wasn't trying to neg her or anything, I was just putting up a silly contrast for comic effect. Also, it is attractive because it shows you are confident enough in yourself not to get defensive when people say these kinds of things.
Obviously if she says yes politely excuse yourself.
I'm not sure what you're getting at here. You would only ask a qualifier like this if you were very sure she was very attracted to you, otherwise this won't get a very good answer at all. It is true that if she puts effort into answering it then you can be pretty sure of attraction, and so you can escalate, but just throwing this out at the wrong time isn't going to do anything magical. You need to have walked her through some easier qualifiers first.
Plus, if the conversation is not going anywhere, this is not breaking rapport, as there is no rapport to break. Concentrate on having interesting conversations rather than relying on random routines to throw out at random.