So in the case of Sri Lanka, was the LLRC set up and subsequently criticised as a mechanism to lend legitimacy to the way in which government forces conducted operations against LTTE? If so, would its mere existence not indicate some level of societal buy-in to the idea that actions should take part according to some judicial form of 'justice'?

The idea to enquire into whether actions were justified in the context of war. Very different from a criminal trial before actions are taken.

You're missing the point, "justice sought by the ICC" implies that the ICC just wanted to execute them, which is obviously not true.