I think you're seriously underestimating the power of motivated cognition. If they're in argument mode, it doesn't matter how reasonable you sound or how politely you phrase your questions, because their goal isn't to clarify a point or to reach an agreement; it's to forcibly make you give up your position. It's as Error put it in the actual post:

When your theist parents ask you, “What? Why would you believe that?! We should talk about this,” they do not actually want to know why you believe anything, despite the form of the question. There is no genuine curiosity there. They are instead looking for ammunition.

Trying to use reasoned discussion tactics against people who've made up their minds already isn't going to get you anywhere, and if you're unlucky, it might actually be interpreted as backtalk, especially if the people you're arguing against have higher social status than you do--like, for instance, your parents. It's unfortunate, but it's the truth. And believe me when I say that I have met people like this in real life. The experience was not pleasant.