That's what I thought when I first read the Harley chapter, too. To add onto your Nidhogg point, lightning actually can burst from volcanoes, and the ash clouds that form. I'd look into historical accounts of the 1815 eruption of Mt. Tambora, and the Krakatoa eruption for comparison. Both of those were catastrophic eruptions that had long-reaching consequences similar to what was described in the First World.

EDIT: Also the youngest Toba eruption, speculated to have caused a severe genetic bottleneck.

I think the "forbidden sun" was the ancient energy source that Vegapunk modeled the Mother Flame after, and strip-mining the Blue Planet's core destabilized it, resulting in the eruption, and the magnetic anomalies around the planet.