Just a couple of points.
Before the 1930s, the British Army formed in Four Ranks, and before 1908 (under the 'old' eight Company system) the Colour-Serjeant (or Colour-Sergeant, if you prefer) was a platoon (half-company) commander - The battalion was reformed into four companies of four platoons each commanded by an officer durting the period of the 'Haldane' reforms.
Deever was a private (hence " I've drunk his beer..." from Files - it just means that Deever 'stood his round' in the wet canteen<1>) the 'stripes' are good conduct stripes (so Deever was an 'old sweat').
And as to why he was hanged? I look to "The Young British Soldier"

" If your wife should go wrong with a comrade, be lothe,
"Don't shoot when you catch them or you'll swing, by my oath.
"Make him take 'er and keep 'er,
"That's Hell for them both,
"And you're shot of the curse of a soldier."

At least, that's how I interpret the poem.

Walrus

<1> In the pre-Great War Army it was forbidden for even a lance-jack to drink with privates, it would cost him his stripe.