It has never been a military decision to deploy armed troops onto their own precious land, it has only ever been a political one. To even attempt a prosecution in such circumstances undermines the trust that the military must have in the very people they serve.
If the State wishes to pursue those in political power who made the decisions to deploy UK Armed Forces in NI, on the basis that shootings would be inevitable, then it should do so. Prosecuting the individuals who were the instrument of political power is a great injustice and a betrayal.
I have no doubt with a sufficient passage of time any military action could be viewed as unlawful. But the means, motive and opportunity were provided by the State, not the soldiers involved. The rest is down to individual judgement in the most testing of circumstances.