How to stay long in China

I'm from Australia and have never been to China before, but want to go there for an extended period of time (6+ months) to live with my girlfriend.

My local travel agency has informed me because I've never been to China before, the best L visa I can get is just a double-entry 6 month with a 30 day maximum duration of stay. If my girlfriend sends me an invitation letter to "visit family/friends", will I still need to provide proof of a return air flight before the 30-day stay duration to be accepted the visa? I don't want to unecessarily buy a return air flight because my intention is to extend my visa in China.

Before the 30 days of duration, can I travel to HK, come back to mainland China to get an extra 30 days of stay? In order to stay even longer in China, what should I do next? Can I apply for a new visa in China that will allow a 6 month duration of stay? How can I get it?

I won't be officially working or studying in China, however I have enough money to live there for a year and have a TESOL certificate to eventually find a job while there.