Just finished reading everything here, and I think what I would say has been covered, but I did want to drop an idea here for the OP...
OP said...
Maybe I'm missing something, but if a creature is hostile towards us and clearly doesn't even want to question us, but draw its weapon, then as far as I'm concerned it's a fight, and negotiations of any kind are no longer an option on the table.
There are times where I, as a DM, have mentioned (or thought) "this could have been handled diplomatically" but for me it is hard to relay that this is an option without killing the suspension of disbelief. I mean, I could say it up front... but that's essentially telling the PC's what to do. I could not draw up the encounter on the mat until the first swing was made, which is a cool idea, but doesn't always work (many times, the players want the room drawn out ahead of time, etc). Generally speaking, I don't like to start combat and then begin negotiating mid-way. I can understand (and have used) the "wait wait, don't kill me" method which works okay, but I don't like to repeat things, so it doesn't occur regularly - maybe it should.
I guess what I am saying is, it's not an easy thing (I would imagine) for most DM's.
With that said, as a player, if the DM was regularly telling me "you could have completed that diplomatically" and I felt like there was never a way to have known that - if I felt that fighting was not my first resort, but was constantly pushed on me, I would be a little frustrated (not much, just a little). I would ask the DM what he/she was expecting - or, to give an example of how we could have engaged in diplomacy during that last encounter, etc - see what they had in mind.
If no ground could be gained there, what I might try (and maybe this is bad advice) is I would start entering encounters without my weapon drawn... I would keep it sheathed even if it felt like a fight was certain... in fact, I would keep it sheathed even as I was getting attacked in the first round, maybe even during the second round, all the while trying my hand at the diplomatic approach. I would try the non-violent approach for each encounter until it either instigated diplomacy or the DM realized that I really did not have a clue of how else to negotiate these encounters, haha.
But that's me pretending to be you in your situation (and pretending to be a bit standoff-ish). As it is, in the games I play even when we do miss out on a diplomatic approach, we generally know it was our fault or know exactly how we closed that window of opportunity, etc. It usually starts with a conversation that is interrupted with an "Oh great, here we go" from one of the players.
---edit---
One more idea as well - don't have the books with me but iirc, you can decide (on the killing blow) whether or not it was a killing blow, or a knockout blow... if i am remembering that correctly, you could state that when each creature drops "oh he falls? well, that was a knockout blow... want to question him later"
---edit2---
Yep, there it is... (PHB p. 295)
"When you reduce a creature to 0 hit points or fewer, you can choose to knock it unconscious rather than kill it. Until it regains hit points, the creature is unconscious but not dying. Any healing makes the creature conscious. If the creature doesn’t receive any healing, it is restored to 1 hit point and becomes conscious after a short rest."
So using that, you don't have to miss an opportunity if you feel someone has information you need
