If you have a mailing address in the USA, use it. Your physical address is what it is, but since you live abroad, you are also going to need to provide evidence of your intent to re-establish domicile in the USA. Do state your domicile as USA though. That's a separate question. Since nothing is going to be mailed abroad, a US address for mailing is best.
Yes, become an A-Student of the excellent I-864 instructions.