Well, the others have addressed your other questions nicely, so I'll stick to the last one. In order of preference, I would say Mercy first, RWH and the Northern very much last (in fact I wouldn't even consider this one). Private won't be an option for you now, they will all be booked out at this stage. But it sounds as though you've decided on public anyway .

Both Mercy and RWH are pretty new hospitals, but the Mercy is bigger. RWH definitely has space issues, and is likely to boot you out to a satellite hospital, like the Northern. Mind you, last I heard, there wasn't alot of choice to be had. You are assigned to a hospital by your postcode, and can't get around that. It used not to be the case, but seems to have changed recently. Your GP should be able to help you, but if not just ring the hospitals and find out for yourself. Good luck