Hi Eryka
I reread the first thread (have to admit I don't remember posting on it - but I did) which was interesting but didn't get me alot further in looking for Cecil.
As you see from Dee's answer SA has no record.
I went looking for the WWI unit as I was wondering about his age. Men lied about their ages on war records - for various reasons - and they were very rarely checked in either WWI or WWII. They usually gave the correct day/month - it was the year that was altered.
He doesn't seem to have the name Lang on the WWI record. (On there as Cecil Trevor mother Mrs G Trevor) On 29 Dec 1914 he lists his
age as 24, which would give a birth year of
1890. The following day he says his age is 21 years and 10 months?? giving birth
Feb 1893. On 21st February 1917 he is Cecil Lang Trevor and gives his age as 25 years 10 months - which gives a birth month of April/May and year of
1891.
I'm not sure if this is relevant or if you checked it before. It leads me back to my original thought - was he born on board a vessel or even in the UK. I think I read his mother arrived here 1890.
The best I see in England is a Cecil Lang registered Dec Qtr 1889 Wandsworth - no Cecil Carrick or Trevor

(Could probably check for this one remaining in England on the census) There is also a birth in NSW Cecil R Trevor parents James and Mary born 1890
You have been searching for awhile - I am not very good at "looking outside the square" but perhaps some others may come up with some different ideas. I hope you eventually find him
Trish