For air BNB you need Owner ID and blue book. I saw some people who were staying in an AIR BNB who did TM30 at pattaya immigration. They just emailed the Owner.
Where does your gf live..? With you..? Or at another residence. If she does not live with you then get a copy of her address, the blue book and her ID card and say you live with her. Or go to BKK as discussed above....Yes TM30 is a real pain...
Oh right. Yes there should be no reason why you can't leave Thailand on a Thai passport and enter Australia on an UK passport with an E Visa and return the way you said....back to Thailand....as far as I know anyway.