So now that I've had a chance to digest, some thoughts.
I was very surprised to see survivors, but the explanation that Chaos was playing with them, drawing out their conquest slowly, makes a lot of sense. I would bet the Slaaneshi even allowed them access to water in some fashion, because merely dehydrating them out wouldn't be as fun.
I wondered how long the Karak Vlag was trapped by their own reckoning, and then I realized that they themselves have no way to know. No sun, no moons or stars, no seasons. Only endless darkness lit by daemonic lights. Sleep only when so exhausted you can't stay awake any long, eat or drink only when your body can take no more. Was that a month or a year that just passed? Who knows? In fact, literally the only firm means of time measurement they might have had is how many inches their beards grew. Brutal stuff.
Perhaps they'll have some people outside the karak and prepared to talk by the time the Expedition comes back this way, and we might get some sense of how many made it through. Tens of thousands is surely too much to hope. Maybe thousands. Maybe only hundreds.
Maybe someday we'll even be allowed back inside for a visit, after a special three day security check. (Shortened from the standard 10 days in honor of Mathilde's status as one of the people who broke the Siege. Not that they'll be any less thorough- just put more inspectors on it.)
I think a visit to Uzkulak is definitely worth it. It'll be a valuable first hand experience of a place few wizards have visited, and if Ranald and the rolls are with us, we might turn up something interesting or informative. If nothing else, that mention of an alternative food supply should weigh heavy. We don't know if the Dolgan will keep their bargain, so as many back-ups as possible is a good idea.
I'm less sure about the Skaven combes, but... Well, Mathilde speaks Low Queekish and can read their writing and is really, really good at infiltration. It might end up doing some good. It has at least as big a chance of doing some good as talking to expedition members, anyway.