I really like this. I come at this from a more NSR than OSR perspective, but I feel like I wouldn't go so far as to say that the difficulties of parleying and fighting should be matched - the situation is what it is. And even if not fighting is always the better choice, the players won't know that and won't always make the best choices.
I generally play these types of games with combat being what happens when things go wrong. Unless one side has a very clear advantage, combat is high risk for everyone involved and usually it's better for everyone to avoid it. Combat happens, not because either the player characters or the NPCs would choose it as their first choice, but because you play games about situations in which everything is likely to go wrong.