So Myles Murdoch Lachlan McInnes got his wife's father's name
OK what I got was she thought her dead husband's father was Lachlan.
Later after much talk she went and got her marriage certificate and it was Myles Murdoch and he seems to have had a third given name and it could be Laclan but she said it was too hard to read (the first two were clear but whoever wrote it was running out of space so he squashed the third name up).
Hi Karen and Meg
Sorry

... but I have to disagree with this new theory, that Myles Murdoch McINNES had a third christian name.
If he did, then
he never used it. It's not recorded at his birth, nor on his NZ marriage record or the births of his children in NZ . Neither does a third name appear for him in directory listings (NZ) ... and (if that's his death in 1928 - NSW) then again ... he's just plain Myles M.

The only place "Lachlan" crops up, is on his son's marriage cert. (but then that is also doubtful, because the name is difficult to decipher).
It is possible that Ernest had a third name and used that and this is why we can't find him. 
Yes ... anything's possible, but I am still more inclined to think that Ernest was adopted out when he was a very small babe (and was given a new identity). He was born sometime between the last quarter of 1876 and July 1877, when his older siblings were placed at Randwick. That institution quite probably did not accept very young children, hence Myles and Ernest were separated from their siblings.
Lu