I have just played one game, but after reading the forum I think almost all my doubts have been answered. The only one that remains is about what happens after a unit retreats one hex and the attacker advance or pursues it. I asume that after the second attack (pursue) the units are engaged, but what happens if the players decides not to attack? And after the avance, are the units engaged?

No engagement unless there is an attack that follows the pursuit. And, no, there will be no engagement after an advance.