The one thing I can contribute is that any acronymnic derivation for Pom is rubbish. If a word has attribution from before WWI (which it does), any acronymic derivation is rubbish. It was only with WWI that they really started to be used (Toc-H, Ack-Ack, and suchlike derive from the then phonetic alphabet). You can debate about the why of pomegranate (the version I heard was that transportees were fed pome's to help ward off scurvy on the voyage), but pomegranate it is, not "Prisoner of..."

It's not "For Unlawful Carnal Knowledge", it's low German; it's not Ship High In Transit, it's A/S scitte (sc was pronounced sh) - common & garden (& messy & unpleasant) diarrhoea!