Update: as stated in the ICE Digests, it is the game state that matters, not individual cards leaving/entering play. So, this makes sense.
In ARV #10, the decision was YES, thereby amending Shelob and Will of Sauron to say "when Doors of Night is not in play" (instead of "leaves play").
A NO decision in ARV #10 would have resulted in the following clarification:
Interpret "when Doors of Night leaves play" as "when any card titled Doors of Night leaves play."
I think Echo of All Joy can be decided consistently with ARV #10 by NOT limiting the discard of Echo to happen only when the card titled Doors of Night comes into play. That is, Peril Returned can discard Echo if Peril causes Doors of Night to be considered to be in play (when Gates is not in play).
We could amend Echos based on Shelob and WoS:
Play on a resource long-event if Doors of Night is not in play. The long-event is not discarded as normal during a long-event phase. Discard Echo of All Joy and target long-event when any play deck is exhausted or when Doors of Night comes into is in play.
This seems to balance the strengths and weaknesses: using Gates (to block Peril and save Echo) vs not using Gates and being susceptible to Peril. Peril being powerful over Doors in one sense because it's not an Environment card (twilight can't cancel).