Perhaps many of those with the ability or knowledge to deal with the plants were dead, injured, trapped, or just missing.
I can see a plant figuring it out if 10% of their personnel aren’t around. But not if 75% can’t be found.
Yeah, maybe but again, this plant wasn't operating in a vacuum. There are other nuke plants within 100-200 miles of this that had people who could understand the systems and what to do to shut it down and keep it cool.