On a side note, yes you can do less than 10 HSC units in total - presumably among other things, you don't get a UAI. A friend of mine did this...
re:repeating yr 11
Total waste of time, in my opinion. Yr 11 marks don't count for yr 12, unless you're doing an accelerated student. In most cases you are far better off waiting till yr 12 anyway, or simply extending your hsc over another year. I know it sounds weird, but the end goal of the hsc is to get OUT of high school and into something (tafe, uni, apprenticeship etc) that will actually get you a job. the HSC only gives you a uai.