Wow, that's a lot of questions in detail.
The middle name is a family oddity, all his other siblings either had no middle name or a middle name taken from a previous maiden name of ancestors in recent line. The McKay is the exception, altho it doesn't appear to be used in any documents once he arrived in New Zealand.
I have no sighted any prison records, what I know of him comes from a booklet written by a distant family member in the 1980s. His father was deceased at 1916, his mother died the following year. Given she died in the middle of the War, it hard to follow the narrative exactly, but four sons died at the time of the War, 3 in military context and one civilian. Her deaths certificate says 4 living son, but the ages don't correspond well to the most likely living sons. He seems to have worked as a labourer when he worked. He was 4/5th born with a twin in the family, the twin died before one year. I will look further at obituaries.