1 - Attacking units stop at the line before entering the space they are attacking. If they win the battle they are not forced to go into that place. They can go to the place they were before the attack. Correct? If not, an overstack (4 units) can occur.
Attacker (if win) must enter attacked space with at least one attacking unit. Rest of attacking units can enter that space or leave at space they were before the attack.
If overstacking occurs in any situation during the game - overstacked unist must be destroyed totally (removed from the game).
2 - Although it is more or less clear: the decks are separated? One side has 1 deck and the other another one. This is not stated on rules.
Yes. There is one Polish deck (55 cards) and one Soviet (55 cards).
3 - The number of SP loss is what determines a great victory (4 or 5), and not the actual number of inflicted losses, correct? If I inflict 4 SP losses but the opponent only can take 2 I will win 1VP for a great victory.
Great victory bonus (+1 VP) is gained only when defeated opponent inflicted in fact - 4 or 5 SP.