Thanks, sparrett, for your observations, all of which are true. I just want to ask about the birth record for Patrick Daniel. Does the registration district 'Marine' indicate a birth at sea? I guess also the registration number 1 would indicate the first birth registration in that district on arrival in April 1920. As Edith and Denis gave the same address on their marriage certificate it's not surprising that a baby was born soon after. A newspaper report of 28 January 1948 gives Patrick's age as 28, so that figures, although I appreciate that newspapers don't always get it right!
My guess is that Edith and Denis met in London round about the time of his discharge from the AIF in the summer of 1918. Maybe they went to Liverpool with the idea of sailing to the USA, as this would be the logical port to sail from. They lived there for a little while, as Denis worked in a sugar factory. For whatever reason they returned to London and went to Australia instead, as Denis was entitled to do as an ex-ANZAC.
I would have thought that the baby would have been listed on the disembarkation records at Fremantle.
By the time of his naturalisation in 1952 Denis Papchuck was still illiterate, in English at any rate.
EDIT: Denis was of course discharged in 1919, not 1918.