Sorry for the late reply, your mother is not a household member as you are not currently living with her. She would fill out her own I-864, not an I-864A. You are not going to the back of the line necessarily, but it is a pausing point for you. If you utilize the search bar you can find loads of examples of what people submitted for domicile, you'll have to use what works best for you. Know that if you keep getting denied then you'll probably end up having to move back to the US, as that's the easiest way to prove domicile. I was able to prove domicile in the US through a combination of things including maintained US address, bank accounts, not establishing ties to Ireland, etc, but you might have to prove that you intend to re-establish domicile, of which there are loads of threads about just that (maybe even a guide too?). Research is your friend, and this site is a gift!
You would have definitely gotten better/more useful information in a more timely manner if you had just made a new thread in the NVC section of the forums. I don't know if you've done that in the time since you've posted this message.
Good luck!