Treat Each Action as Your Last is fulfilled after your opponent plays their second action card in a turn. This falls in the same resolution window as "after play" abilities, and your opponent as the active player chooses which order to resolve these in. This means it's possible for your opponent to choose to fulfill Treat Each Action as Your Last before or after the played action's Play: ability is resolved. Any bonus icons on the played action would still need to resolve first, as those bonus icon resolution happens before the "after play" resolution window.
The played action would not be put into its owner's discard pile until after all "after play" abilities have resolved, including the fulfillment of Treat Each Action as Your Last.
The played action would not be put into its owner's discard pile until after all "after play" abilities have resolved, including the fulfillment of Treat Each Action as Your Last.