I personally have fasted for 28 days. The dangerous part isn't micronutrients, it's electrolytes. Fasting for an extended period can put you at a dangerously low level of certain electrolytes like phosphorus, which upon refeeding, are taken into the cells, which results in levels low enough to cause death. This is why extended fasting regimens should only be done with medical supervision.

Also, the initial onset of ketogenesis from fasting is very rough for people who aren't used to it. Most people's bodies are not used to it, and they won't be able to think clearly, they won't have much energy, etc.

I believe the definition of micronutrient covers electrolytes (including phosphorus), but I see what you're saying. There would definitely be challenges!

I wonder suspect this would be alleviated if survivors managed to get like 300 calories a day of various foods, which seems more likely than pure starvation.