I did Heritage for Yr11.
They ASSUME that you can communicate in basic Mandarin and have a grasp of basic characters.
It is COMPLETELY different to continuers course (I do Japanese Cont./Ext) where they teach you Sentence structures and basic grammar from a grass-root level. They DO NOT do this in Heritage. It is assumed knowledge.
Heritage and Extension courses are very similar in that you study ISSUES relating to the modern world. For Heritage you can look it up...I've forgotten it all. There is not set text for study, unlike Extension. Furthermore in YR12 there is a Personal Interest Project that is used in the speaking-component (interview) in the HSC exam.
To OP: Do heritage WITH extra units. It is not suited for some people.
However I would like to point out that many people who SHOULD be doing background sneak into the course. This is inevitable with the Asian languages as there is such a grey area - and people lie, too.
Anyway try it out but have extra units to backup.
Ps:
I am a native Cantonese speaker and Mandarin was completely assumed of me. I think the whole 'dialect = main language' is a pile of bullshit and you cannot assume that. Fortunately my teacher (who has now retired, I have heard) spoke all main dialects (Mandarin, Wu, Cantonese) because he had lived in these places before (Hong Kong, Shanghai) etc. and understood my situation...but I could not compete with people who spoke Mandarin at home so I dropped it