Brisbane > Hong Kong > Hangzhou > Hong Kong > Brisbane qualify for the 72hr transit visa?

Sorrry, as you enter from HK to Hangzhou and then back to HK, you don't qualify for the 72h free transit. Why not leave from Hangzhou for Macau instead?

There's a meeting to attend in HK on the way back. Could we go from "Brisbane > Singapore > Hangzhou > Hong Kong > Brisbane" instead? Does it matter if you are spending two nights in Hangzhou and only one day in Hong Kong? i.e does your stay in the onward country have to be longer than your transit in China?

Yes, this route is eligible. Your stay in the onward country does not have to be longer than your transit in China.