No, you cannot apply for a Prospective Marriage Visa if you are both from overseas. At least one of you needs to be an Australian Permanent Resident or Citizen. Furthermore, your partner cannot be in Australia, when appplying for the Prospective Marriage Visa.
If you are both from overseas and non-residents or citizens of Australia, you will most likely apply for a 3 month Holiday Visa. Please note that you will not be able to work in Australia whilst on a Holiday Visa.
Yes, provided that the marriage ceremony is booked as early as possible, when the partner travelling from overseas arrives. This will allow maximum tine for processing the papers with Birth Deaths and Marriages.
Yes, the partner who is on the Holiday Visa will be unable to work whilst in Australia, whilst the VISA is only for 3 months, so the marriage would have to be solemnised as soon as the partner arrived, in order to allow for marriage processing time.