E Commit, discard X momentum: If this attack deals damage, your opponent loses X vitality and you may ready 1 of your foundations that you have not readied this combat phase.

If this is used on an opponent's attack and the attack kills Demitri.. does the effect still resolve, resulting in a possible double KO? Or would it resolve, lowering a player's vitality in a multiplayer match?

if the attack deals enough damage to D to kill him, the game is over in a 1-on-1

multiplayer, the effect will of course resolve as the game continues until there's 1 player left (or however you decide a winner).

The game ends immediately when a player hits 0 vitality (barring a specific effect like "Finesse" - though that's only in Legacy). Sorta like cycling, where the game just STOPS when it happens.