MotW “solved” it by having completely non mechanical backgrounds, and using a stat-based move for Helping. It is more team/mission based than most PbtA games, though, and doesn’t lend itself (or need) a complex relationship map.

I really like the tech embodied by Masks’ first mission questions, where the team establishes how they came together. I’ve also played around with using flashback scenes to help flesh out connections in a one shot. There may be some room there for a directed set of questions for bringing in a new character. This doesn’t answer the potential disruption to the fiction as established, though.

You kind of have to inoculate against a missing person derailing events because too many story or mechanical connections have been established to that person. Encourage folks who are isolated from each other to have scenes together so they have something to fall back on. Spread the strings around.