I agree Curufin should not be doing anything to actively get rid of Maedhros.
I’m liking many of Haakon’s suggestions for the parley. I’m not liking the suggestion that we show Maedhros acting honorably here.
I think part of the point, in the way Tolkien consistently wrote the parley, was that Maedhros should have acted honorably even towards Morgoth, and that what he did was wrong: he broke the agreement and broke his word, and the promise he gave the emissary was a lie. “Each purposed evil to the other” is not an endorsement of Maeshros' behavior. Both sides came to the parley with more forces than they agreed upon.
Also I think that none of the brothers are foolish enough to trust Morgoth. Because they know that obviously Morgoth intends to betray them, they don't have qualms about betraying him too. (A lot of otherwise honorable people would not go as far as Faramir in saying "I would not lie even to an orc.")