If I choose a card with Ambassador, before I decide how many of that card to put back, my opponent needs to approve a undo request for me to change the card chosen.
I recall that a similar issue happens with Hamlet when choosing "discard for action" vs. "discard for buy" vs. "discard for both". The permission of the opponent should not be necessary to undo these intermediate steps in resolving an action.
Agreed. I hit the wrong key often enough with Hamlet to want that auto-undo function, in these mid-turn, no-additional-info-received situations.
+1