I wouldnt go to parttime hours if you can aviod it as it will probably affect your maternity leave payments in the way you described.

Can you take sick leave instead, or use temporary disablilty - I know one mum who used her supers temporary disablility cover and was on 70% full pay for the last 5 months of her pg.

The returning to work part, relating to paid leave and whether you'd have to repay it, is something your employer will have a policy on. When I took parental leave at my old employer is was 12 weeks paid. There was no requirement to pay it back if you didnt return to work.

As far as having another baby/becoming pg while on maternity leave, I think the entitlement to 52 weeks unpaid leave is based on 12 months continuous *paid* employment - I think you need to be back at your job for 12 months before you are entitled to another 52 weeks? But I'm not sure.
Unpaid leave is legislated so I'd ring the relevant government department to enquire.