I want in (to Australia)

I am a US citizen currently living in Australia as a dependent on my mom's working visa. When I turn 22 next year I need to leave Australia (because I will no longer be considered a dependent).

I will have a BA with a double major in English and Psychology when I leave, from an Australian university (I'm in my third year). As far as I know, that doesn't qualify me for any of the skilled occupations, but I may be missing something. I cannot afford to continue my education.

I work at a fast food restaurant at the moment, and I'm not sure if they would sponsor me just because any 15 year old in high school can do my job.

I've met an Australian who I love very much, and we are looking for ways for us to stay together. I am living interstate while I go to school with my partner, but I am still dependent on my mom for living and tuition. My partner and I live in the same house while I'm at school, we've just opened a bank account together and we've been together for 4 years. We have looked into defacto and even getting married, but we don't want to do this if it's not necessary.

I had a bit of a scare when I called the immigration hotline because the woman on the line suggested that I'm not here legally because I don't live with my mom while I'm in school. I know that this is not the case, because my mother's company does routine checks to make sure we're all allowed to be here. There are others like me who are also dependants who live interstate while in school. But since that scare, I have been reluctant to speak with immigration again.

My questions:

1. Who can I speak to about this that won't cost me an arm and a leg. I'm extremely poor (paying international tuition will do that), so I can't afford to spend any more than $1000 (but preferably less than that) on finding the answers.

2. I am concerned that if I apply for defacto, that will suggest that I am not dependent on my mom (thus indicating that I am not here legally). Can I apply for defacto if I am a dependent?

3. How might I go about marrying my partner or applying for defacto while I am on my mom's visa? Can I do this without leaving Australia?