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This one involves discard effects & the newly spoiled neutral water province (By Onnotangu's Light).
For the purposes of this question the board state will be:
Player 2 has a Border Rider with 2 fate and an Embrace the Void attached.

Conflict at "By Onnotangu's Light".
Player 1 plays Assassination, killing the Border Rider. The Border Rider now leaves play, triggering the leaving play steps.
Based on these happening simultaneously, I expect that the fate will not be discarded and ETV will be discarded before it's able to trigger since the lasting effect is still applied while these steps are happening. Once all 3 have finished happening (and the lasting effect expires), then we'd be able to remove fate from the character, except that at this point the character is already in the discard pile and is no longer "leaving play".
This puts the game in a bit of an unusual state. Does the character have fate on it in the discard pile? What happens if Player 2 now plays Cavalry Reserves to bring the Border Rider into play?

It’s not quite as complicated as all that. The fate will still be returned to the token pool as cards in the discard pile cannot have tokens on them. You are correct that Embrace the Void cannot trigger during a conflict at By Onnotangu’s Light.

[Tyler Parrott, Oct 20 2020]