Say if I a ghost sim got hit by a random lightning or a stray meteor will they go back to the Netherworld? or will they just become singed?
I'm aware that they're basically already DEAD, and does not age (at least in my game) but can they survive accidents that a living sim can't?

I lean to the idea them being singed than them returning to their graves. They do the wisping thing when it's past they're staying curfew (daylight), aside than the event where as mortals that just died and have met the grim reaper.

From having a ghost become a tomb raider (or go back after dying from the mummy's curse as the case was), they can become singed (and soaked) from traps but otherwise I haven't had a ghost get electrocuted or catch on fire. I might have just been playing boring ghosts. But I believe the only thing that will kill a ghost is by them clicking on their tombstone in their inventory and choosing to 'go back to the netherworld'.