Grossman is American and their military training differs from ours greatly, so unless you can provide a UK study based on UK military training methods, Grossman is irrelevant.
I don't think you can even begin to argue that UK service personnel are removed from normal mental functionality. If that were the case why are they allowed to walk around and be members of society? Why are they allowed to reproduce, have charge of training others, run large budgets, drive cars, have mortgages, responsible jobs etc? You make it sound as if they should all be locked up in secure units, and quite frankly I find that insulting.
You may also like to look at the mental conditioning that your dp is receiving via his teacher training. It is there and you will undergo much the same when you do your PGCE.
'With the reports of racism and sexual descrimination as well as prejudism against homosexuality that surround the forces, I do not think they are that well trained as a whole to be tolerant. ' It's prejudice, not prejudism. The Armed Forces are very tolerant - they have to be; it's an offence not to. HM Forces are a microcosm of society so there will be some who have personal prejudices, just as there are in society. Sexism and racism - the women and ethnic minorities who have made the higher ranks give the lie to that. My dh's current boss is a Colonel (female) whom he respects and is happy to work for because she knows her stuff. HM Forces are more tolerant than certain sections of the civilian population.
The whole thrust of your posts to date is that you consider the military to be mentally disturbed, mentally conditioned to kill, and unable to relate because they (according to you) lack normal mental functionality. You also repeatedly post that they are unable to relate to people - so you are saying that they are unable rate to kids, who are people too. You choose to ignore posts from those of us who have a damn sight more experience, very close up and personal, of the military and their abilities, and some of us are also teachers, so we can see how having had a military training could be effective in some schools and some situations. As SFP points out a 22 year old cannot always relate either - and is neither old enough nor has enough of a presence to make an impact. Many of us I suspect are also considerably older than you so are viewing teaching and education now from a wider perspective than you have, as we have have been able to look at education from our schooling, our teaching in some cases, and what we see in the schools our own children attend.
For me, it's the sense of can do, not giving up, team work, leadership, positive mental attitude, wanting to achieve one's potential, setting high standards and self discipline that exemplifies the Forces and I believe that that skill set would translate well to the classroom. It's a shame that you can't see that.