Can units be intentionally overstacked, in a sacrifice play?
Example: In order to save an unsupplied 5-3, can the Soviets move a 3-3 adjacent, then have the 5-3 move onto it, with the intention of sending the 3-3 to the Shattered box at the end of the movement phase, thereby saving their 5-3?
Excellent question! YES you can absolutely do this.
In fact... Look at the example of play and quiz on page 18 of the scenario booklet of the game (Save Smolensk and the Bryansk front): That exact case is pictured and explained.