Declaring it one turn in advance is the right answer.
Take the deceleration of TCS Orestes in Scenario 2 as an example: Pip declared he would slow down to speed 2, so in the first turn the "Last Maneuver Performed" was "Speed 3, decelerate (1/2)" - in the next turn, it was "Speed 2."
Yes, that's absolutely possible. At an acceleration rating of 1/2, you'd declare the deceleration in the first and the acceleration in the second turn, ending up as fast as before. For 1/3, it takes a bit more time, depending on how long you've been accelerating/decelerating/turning. If you just started turning in the last turn, you can simply counter it in the next turn by ordering a turn in the opposite direction. If you've already turned to the left for two turns, you'd have to turn to the right for two turns as well.