Dear Ian,
Some Army Lists are available on cd - here's the link to the one for August 1918[added benefit over the original - searchable!]:
http://www.armylists.org.uk/wherecanibuy.php?id=3880None of the Harrisons had an initial letter A for their forenames - nor any possible variation for Albert or Edward. Seven of the eleven had service in the ranks, dating back several years - so clearly not possibles for your man. One had a commission dated in 1914, again impossible for your man, another dated from 1916 - ditto - and the last two were born in 1899, so unless your man looked incredibly young - also impossible.
I'm not totally sure how one became an officer in 1917-18, but, as usual, the GWF has some basic info - see:
http://www.1914-1918.net/training_officers.htmI don't think that he was sent to prison - there's no mention of it in his papers, as there surely would have been, and any prison sentence would, I strongly suspect, have made it impossible for him to gain a commission in the British Army.
Another possibility has just occurred to me - triggered by your idea of a "picture in my head that he would turn up at another AIF camp and just start fighting and joining in." - Perhaps he did just that. Enlisted in either a British or Australian unit as a private and, because of his knowledge and ability, got himself put forward for officer training? I reckon that would take him about a year, gets him from the Autumn of 1917 to the Autumn of 1918 - or am I just being too fanciful?
It would, perhaps, be useful to know what the AIF were actually doing about him from the court-martial onwards. Did they really just turn men loose? Was there a procedure? Do we know what happened in other cases of a similar type? - I doubt if your man was the only man dismissed the service by the AIF in Europe. Why was there a year's gap between the dismissal from the service and the official discharge? If a man had been dismissed the service, but not discharged, what was his status? As he had been dismissed, why then did he have to ask to be discharged? Armies tend to do things by the book - so what did the book say?
Given that there were two occasions after he was dismissed when the AIF had some form of official contact with your man - once when he applied to be discharged and again when somebody knew that he had entered the British Army, then presumably there was still some kind of official AIF process going on - they had not cut him loose completely. It also occurs to me that Harrison himself would have been unlikely to tell the AIF that he was now a commissioned officer in the British Army - so that information would have to had come from the British Army, which, if correct, means he was using his real name.
All that having been said, I'm not sure that it gets us very far. I shall be back at Kew at some point next week and I shall have another go at trying to find something. I'll start with the same list as last time - this time looking at the Lieutenants, and then see where we go from there.
jds1949