1. When forcing retreat, what does any hex means? The "retreat" part kind of sounds as if it would be backwards, but is it legal to move the opponent say one step forward instead?
Into any vacant hex is valid, even forward. Your attacker can them move into the hex you vacated and change facing if he/she so desires. If no vacant hex exits, you must make a 3d save vs DX or fall down.
2. Dodge and Defend. If you have lower adjDex then your enemy, he has probably already struck/shot you before you choose this action. So I'm guessing it's only viable if you're faster than him, cause I can't find anything about it lingering to next turn or anything like that.
You all play thru in adjDX order, however, you can say "I'm dodging/defending" at the time of the attacker's move before he rolls to hit. This means that when your adjDX turn comes around, you've already taken your action and can't attack.
3. When disengaging with a higher adjDex then your enemy, he may attack you, with a negative effect equal the difference in your dex. Does this happens as you Disengage (as a free action?) or can he strike you at his turn, even if you are one hex away?
The opportunity for the higher adjDX person to attack you requires that they be engaged with you before you take the action; the attack is as you disengage from being close to them.