I'm not sure this 'argument' can ever be resolved. I do wonder if its significant, though, that the Mouth of Sauron claims he is 'a Herald & Ambassador & may not be assailed'.
It seems to me that he is attempting to make use of some kind of Middle earth equivalent of the Geneva Convention, & clearly expects Aragorn et al to be bound by that. If it is the case that heralds may not be assailed then it seems likely that they would not take a direct part in the fighting, or that kind of 'diplomatic immunity' would not have applied.....
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