I can provide you with my save if you don’t mind once I find it. You need to finish this quest before everyone dies and.before interrogating talk to Radzig. And don’t postpone the cure. If they die anyway, loot them before their corpses disappear.
I have heard about a bug at Questions and Answers.
And “You should not resolve(or fail) Pestilence AFTER YOU GOT THE LETTER”
Because the end of that quest may try to set an objective to get the letter already you have and you stuck.
You should run to sir radzig immediately after you get the letter without any other moves.(Pestilence have time limit)
I guess that was your problem.
You can resolve the Pestilence after you bring the letter to sir radzig.
(or you shouldn’t get letter till you resolve Pestilence.)