Mrs FB and I had "results within 1 hour" antigen tests on Friday. Mine came in 20 minutes, hers came in a little over 1.5 hours. There was 2 minutes difference in our sample taken time. Beware.
The test needs to be done 2 calendar days before, so you have more than 24 hours.
The day 2 PCR doesn't get 'linked' to your passport as such, its more so just a link to say that you are the person that did have that test as your passport gets checked when you do it. Of course if you use a mail test it's less important anyway. Surely when you enter Australia, if you had your pre-departure PCR test (for Aus?? It's not clear, sorry) on a different passport you could just show that if requested, or your concern the airline won't let you board if it doesn't match the one you're using to exit the country? Are you planning to use the Day 2 PCR as your pre-departure test for Aus?