Having the NPC "arrive suddenly to be helpful/unhelpful to the PC" is pretty vague. What constitutes "help" and how far does it go? How much authorship do you want to give them? If the PCs get into a bind, can one of them just drop a hold and say, "Um, Rolfball shows up in his tank. Yeah. And with his entire gang, too." Also, as written now, each player is naming one NPC in the settlement - do they have to spend all of their hold on that character? How does that work if someone knocks it out of the park and holds 3? Might they not want to spread it around?
Also, when you say "pre-create the settlement," do you mean that you (as the MC) already know which NPCs are present and what their various roles and relationships are? If so, I don't think you need to do anything more than follow the MC principles and honestly answer their questions.
To take your example, say the players roll into a new settlement. They variously roll+Hot, some of them have hold, some of them give the MC hold, some a little of both. When it comes time for the Gunlugger to get a new bang-bang, the player is going to ask, "hey, where can a guy get a decent gun in this shit-hole?" And you'll say, "Oh, yeah, you know that Dremmer is the guy you want to talk to. He's got a bunker full of guns and ammo right on the edge of town." And it's entirely possible that one of them (say the Operator) might spend a hold right then and there and say, "Ah, Dremmer, right. I did that guy a solid last time I was in town. We're cool. C'mon, we'll go see him and I'll set you up." This is your golden opportunity as an MC to follow this where it leads by asking probing questions: "OK, what was the nature of your work with Dremmer last time you were here?"
You might occasionally want to be a kind MC and remind them if they have hold to spend when you introduce a new NPC. You can even ask them temptingly - "anyone want to spend a hold to be on ol' Dremmer's good side?"
But if none of them bite, maybe when they jander into Dremmer's Bunker O' Guns (tm) you drop one of your MC hold - meaning Dremmer's going to look up from the counter at one of them and say, "You again?!? What the fuck do you want? And before you say a single word, know that if you come anywhere near Maggie, I will cut you."
And if one of your players doesn't drop a hold right then and there at the first mention of her name to get +1 with Maggie, then I question their dedication to causing trouble. ;)
You can also use a hold system like this to link the players into the "existing" politics of the settlement organically. Which NPC(s) in town hates Dremmer? Maybe Keeler, the local black-marketeer. So when you first introduce Keeler on-screen, just cut to the chase, drop one of your hold and have her say, "I hear you've been getting pretty cozy with Dremmer. You might wanna watch that. You wouldn't want to get tarred with guilt by association, wouldja?" And then explain the nature of the negative association - "Yeah, that work you did for Dremmer a while back? Keeler came out on the losing end of it. She's not 100% sure you were responsible, but she obviously has her suspicions."
Or turn it around on them. "You walk into the joint and Keeler, the local black-marketeer is giving you the stink-eye. What did you do to piss her off?"