In regards to Astral / Umbral Eras...
What constitutes the fall into an Umbral Era is a Calamity, cut and dry.
What constitutes the rise into an Astral Era is... decidedly less so. The GC leaders declaring the 7th Umbral Era was over was, yes, little more than a PR move not based in anything scientific because there's no known scientific measurement for what marks the transition.
(This was mentioned in an interview somewhere; I can't recall which one or have a source, but I do remember this.)
Well... in fairness, conflating Hydaelyn-the-planet with Hydaelyn-the-goddess might be just a bit of a stretch. It makes logical sense, but Lahabrea speaks of them as if they are separate entities before engaging us directly in the Praetorium. It's Lahabrea so he can't be fully trusted - duly noted - but it's there.
"Like a parasite, [Hydaelyn] must be burned out if the planet is to recover."
-Roughly. Perhaps we'll see what he meant.
My original way of reaching that hypothesis was by using the crystals of IX as a base; though they can't speak, they are the terminus of all souls on a given planet. If Hydaelyn is "all made one," it would stand to reason She serves a similar purpose... perhaps the point of Calamities, then, isn't to cause planetary devastation, but mass death - create an influx of souls so great Hydaelyn simply can't process them all and the strain breaks down one of Her walls?
Hmm...