During the fight with Evelyn an apparently critical NPC died, namely a zombie Jake.

After continuing the game and solving quests for some time and eventually (so far) coming to the conclusion that I'm somehow stuck in the main quest, I found out online that the dead NPC was actually crucial to continue the main quest and was not supposed to die. Starting over isn't really an option.

Any solutions or workarounds? I can still see the pile of ashes with Jake's name on it where he died, but resurrecting doesn't seem possible.