Hi from me too Cath,
Just an observation here.
Mary Jane (nee DONOVAN) and her husbad were into what was considered then, advancing years.
They are unlikely to have reached out to a stranger to adopt a baby boy.
To my thinking the mother of the child is likely to be one of the daughters born in the 1870's.
Mother Mary has stepped in to the rescue! It is a well known story. 
I have no evidence to support this, just conjecture.
Sue
That's what I assumed too and it's entirely possible.
Other evidence of this possibility, that I saw, was the fact that Emily (daughter of James and Mary) went back to Perth while the other family members stayed in Adelaide. After the lady I spoke to about Leslie Smith pointed me in the direction of the Adelaide Hospital records, I found an entry for Emily Nelson right age etc who was admitted for something called "Endometritis". Not to be confused with Endometriosis, "Endometritis is an inflammation or irritation of the lining of the uterus" and was something that was common after birth or miscarriage. Her admission on this occasion was in 1896 so probably too early for Leslie to be her son on this occasion but perhaps an indicator as to whether she was sexually active. (No judgement!). Perhaps she left Adelaide after an incident of this kind.