Re: It's an 'underworld' festival
Maybe for neo-pagans.
Not so much for Celts, who don't seem to have had the Greek concept of an underworld, but many "Otherworlds" only maybe some or one associated with the dead.
Certainly it was more important than the equinox.
All the (four) Celtic fire festivals were originally about 1/2 way between Solstices and Equinoxes. Various theories.
Yes, America imported Halloween and corrupted it. Turnip lanterns were fine, there are different kinds, the largest are near football sized and there are others that are no use for lanterns being only a bit bigger than a sliotar (a little like a cricket ball used for Irish ice hockey, which doesn't bother with ice). Harder to dig out turnips, but at least the waste goes in stew. Pumpkin has little flavour in comparison. The important thing was the bonfires, the apple ducking etc, not shop bought "candy" nor going round the houses begging.
In the 1960s the fireworks were still legal in N.I. (made for England's Guy Fawkes) but illegal in Ireland (Republic). England seemed to do Guy Fawkes rather than Halloween (Celtic) or All Saints Eve (the too Catholic appropriation of it) then.