I don't know for sure about your first point but I thought it was having homosexual tendencies which was an offence, not just engaging in such activity. Even so, it seems that a 'three wise monkeys' approach was usually taken unless circumstances arose where it couldn't be ignored and action had to be seen to be taken.I can't be sure but in the UK I don't think it has ever been against the law to simply be gay, but rather if you engaged in such activity.
On the general matter of being in and being gay again I am not sure it was an offence simply to be gay. I think it was more an admin matter than a charge so perhaps SNLR. Even back in the day it would seem improbable that someone could be found guilty at CM where they had NOT done something (or at least not planned or conspired to).
The underlying attitude of those who were keen to find and sack was driven by homophobia rather than a simple fear of the blackmail risk. Interestingly, having heard some presentations on the matter, before about the 1950s it was more of a morality issue and homophobia was not so evident.
From those who I have spoken to so charged it was a Spanish Inquisition type process and once accused the search for an admission or evidence was relentless and brutal, such was the prevailing paranoia/homophobia. I think the blackmail idea alone does not simply explain why homosexual acts were dealt with by CM and MCTC, other activities that could just as easily expose a Soldier to blackmail were not dealt with by any such draconian response, other than homophobia then I can't think why that was the case.
I agree that the risk of blackmail was probably less of an actual issue than claimed. Perhaps homosexuality was seen more as an 'unnatural act' and the fear was of a breakdown in trust and cohesiveness.
One thing we can be fairly sure of, the military has pretty much been behind the curve when it comes to most things when compared to civvy street, including equality and diversity. Maybe the military hierarchy has struggled with, and perhaps continues to struggle with, balancing the needs of being a unified force having a particular group ethos compared to the way civilian life has become more about individuality and the needs of the individual above the group, when it's those individuals the military needs to attract to maintain its existence. Just a rambling thought.