There are a number of deaths in Greenock that could be Margaret. I had a quick look for an immigration for John and James, tricky as such common names. Will have another look.
Challenging for a widowed man to immigrate to Australia from Scotland with a young boy.
If John was registered to vote in Melbourne in 1912, the boy would be no older than 10 or 11 years at that time.
This makes me think they may have come as a family unit and she left/died/ after so that he is seen with "Florrie" by 1914.
Noting that when John Brown McLachlan married in NSW, on the Parish Register he has taken a couple of years off his true age. He was actually closer to 30 than 26. His bride 18 years.
Sue
ADDING further observations.
James Brown McLACHLAN, a labourer, lives at his father's 1 Gladstone St. address in 1927 and 1931, the same year as his marriage. His marriage certificate, as noted states he is a manufacturer, so another porky

Noteworthy or not?...the certificate also show that Edith's father is deceased by the note "deceased" next to it .
There is no such note beside James' mother's name.
Sue