Hi Aimie
I did read somewhere that less places in Perth were offering sponsorship, as for the working hours, for PR you have to work for 12 months out of 24 months at 20 hours... I was in a similar situation as yourself in that I didn't meet the requirement due to maternity leave and a period of illness. We looked into the 457 visa and decided it wasn't for us. I just had to put my plans on hold and get on with the year at 20 hours to get PR. If you decide to do this, you will find that the year passes pretty quickly - with having to do IELTS etc for nursing you could be working at this during this time (depending on where you live this can be booked up well in advance), and even apply to the state nursing board (they are all joining together to form an australian nursing board in July), this would leave you in a position to do a modified skills assessment with the ANMC. I know it is frustrating, but there is so much that you can be doing in the mean time, like collecting documentation together and just generally getting your head around the whole process.
Alternatively, if you are not 100% set on Perth, there are places in QLD that are still offering sponsorship on 457's, as far as I am aware your hours worked previously do not matter for that visa, but you do have to work full time once in Australia on this visa.
Oh and I noticed that there are some more of the "Health Jobs around the World" Expos happening in March, Visit Expo - Health Jobs Around the World Expo you might be able to find out if any WA hospitals will be attending that.
Angela