Your final paragraph about using a PC-NPC-PC triangle for this purpose is along the right lines, I think, but I'm not sure exactly what you're describing. Can you explain a little more, or illustrate with an example?
Sure, how about the following:
Our PCs are Diamond (the Chopper) and Bish (the Angel). We know from the Hx round that Bish once stood up to Diamond, gang and all. The questions asked about it at the time have established that Bish was doing some do-gooder shit, and kept Diamond and her gang from killing a group of sick travelers who'd showed up at the hold's gates last summer. We also know that the standoff did not end in violence between Bish and Diamond, (as neither opted for a beatdown), but that's it.
So we throw an NPC into this mix, let's call him Dog-Head. Dog-Head is (now) one of Diamond's more dependable lieutenants, but back then he was just another fucking savage in the pack. So we start a scene with both Diamond and Bish, only Dog-Head's there as an extra. We describe him as sullen and testy and looking obvious daggers at Bish. Maybe he doesn't say anything, maybe he does, that's up to them if they decide to engage with him (Diamond if she says, "what's your problem, asshole?" or Bish if he says, "something I can help you with, Dog-Head?" or whatever). Let the tension simmer and see what they do with it.
But even if they don't (or maybe even if they do), once they part ways, Dog-Head finds a way to corner Bish afterwards, and he's clearly pissed. He says something like, "I don't know what the fuck you have over Diamond, but I haven't forgotten your little episode with those fucking plague-riddled scavs you let in last summer." <--Here's your chance to ask more about how that situation went down, what did Bish do to convince Diamond to back off? Then maybe put your bloody fingerprints on it: "Jalopy would still be alive if you hadn't brought that shit home to roost." Maybe announce some future badness (both for Bish
and for Diamond)? "Diamond might be too weak to put you down, but sooner or later I'm gonna get me some payback. Believe it. In fact, give me one good reason why I shouldn't gut you right here, right now?"
How Bish deals with Dog-Head here (what he says, what he does, how they interact) is going to inform that situation, and is ultimately going to inform the historical situation between Bish and Diamond as well. Similarly, how Diamond deals with Dog-Head threatening Bish is going to add further detail - like who is more important to Diamond, Bish or Dog-Head?
And because the original situation chosen during Hx didn't result in a fight, that probably meant that Diamond had to impose her will on her gang, which might have left some simmering resentments - it certainly has for Dog-Head! - which is in turn going to inform the situation between Diamond and her gang.
Best of all, it encourages the players to talk about the situation, both in and out of character. Everything they say is going to flesh out and reinforce their history.