Your “Commander’s unit” refers to the current unit that your Commander is in. If an effect moves them from their original unit to another, the new unit would be your Commander’s unit.

If a Commander is destroyed and subsequently redeployed via some method (or in situation like Jaqen becoming a copy of that Attachment) then, as stated above, whatever unit they are currently attached to is your “Commander’s Unit”.

If the Commander is not in the unit, for any reason, then it’s not your Commander’s Unit any longer.

Q: What happens if one of the “Simultaneous Actions” Triggers is due to a Game Mode ?

R: In the case of Game Mode triggers they should be played as friendly to the player, eg, they choose the resolution.
In this case the active player would determine if they wish to resolve the Game Mode trigger or the Tactics Card (which, unless some strange event, they should probably choose the Tactics Card).

Q: What happens if an attachment of a combat unit dies during an attack and the unit survives, but at the next step of the attack (panic test), the unit dies also, and is revived with D3 heal by any effect ?

R: Only the last attachment, being the last potential model removed, remains, and (should you roll well enough) a further 1-2 standard Infantry models are returned. The 2nd attachment was destroyed during a previous event/step. This would be no different if some other effect killed a Character during an Attack (such as Expert Duelist, for example).

Q: A melee attack allows a unit to change facing then shift to align. Can a unit shift without changing its facing to face the attacker?

R: Yes

Q: What happens exactly when there are 2 same triggers, like “After the attack is completed” ? Example : Bastard’s Girls ranged attacked a unit of Guardsmen, triggering their order “Charging Volley” and at the same time triggering “Lannister Supremacy” on the Guardsmen.

R: Any and all effects are only calculated when they resolves. There is no case where something will trigger and “lock” anything prior to resolving.
In this specific case the Attack will be resolved and we assume Lannister Supremacy is used, as is Charging Volley. Since this is a case of Simultaneous Triggers the active player (usually the Bastard’s Girls) will resolve their effect first, which is a Charge Action in this case. Once, and only once, that Order has been resolved entirely will Lannister Supremacy then resolve. Note this is a corner case, and what can be said to be a “strange” interaction, in that there is a potential that the Guards are destroyed by the Charge Attack. In this case however the Order was still triggered and must resolve, resulting in a Panic Test at -0/+0, despite the unit being destroyed.

As soon as the final Activation in a Round is completed the Activation Phase ends and the Clean Up Phase begins. Eg, there are no additional “Start of Turns” upon completion of the last Activation.

Q: How does abilities of those 2 units interact ?

R: The dragon reads “hits from this attack can not be blocked, if this attack generates any hits, instead of rolling defense dice, the defender suffers d3+3 wounds and 1 enemy in long range becomes…”.
Since the dragon attack inflicts wounds, the giant ability is not relevant, for wounds are not unblocked hits.

Q: How does abilities of those 2 units interact ?

R: The Scorpion reads “For each unblocked Hit, Cavalry, Monster, and War Machine units suffer 2 Wounds.” Giants read: “This unit only suffers 1 Wound for every 2 unblocked Hits from any Attack or effect”.
In this situation the Giant’s Ability takes precedence as 2 unblocked Hits were not generated.

Q: Can a skinchanger deploy the bear when it’s unit activates, then get moved to a new unactivated unit with Regroup and Reform, and deploy the bear again when that unit activates? If so, what exactly would happen?

R: The Bear was selected, it was deployed. Even if the Skinchanger is moved and re-Activated, the Bear itself cannot re-deploy itself (as nothing is allowing it to do so) and it has already been deployed.

Q: Is this ability triggered by a Charge Action, since it is specifically written Attack Action and we already said Attack Action is not a part of the Charge Action ?

R: No, it does not trigger on a Charge Action, as it is written.

Q: Since activation is mandatory, Gilly would always have to activate Round 1, is this correct ?

R: Gilly can only be activated once per game, but she is missing her (crucial) bit of text about not activating otherwise (eg, specifically there to avoid the issue you mentioned, with the standard game rules forcing her to Activate Round 1 and then never again)

Q: If a unit is both Weakened and Panicked, can Adaptive Planning be used to change the Weakened condition into the Panicked condition (which the unit already has), thus leaving the unit with only the Panicked condition?

R: Yes

And Now His Watch Has Ended triggers upon a unit’s destruction. It is not limited to Combat Unit or Non-Combat Unit; both are Units.

Yes, multiple instances of Now His Watch Has Ended can trigger off the destruction of a single Unit.

Q: If Jeor NCU moves Donal’s Defensive Formation to another unit with his claim ability from unit A to B, when does it wear off? When A activates as initial target or B since it’s the one holding it?

R: The effect of Defensive Formation would check the unit it is attached to. Eg, it would be removed when the newly attached unit Activates.

The trigger on this card should be “When a unit would be destroyed” to match all similar triggers. This will be corrected in the next Errata release.

Q: In this Game Mode, when choosing objective cards before deployment, it states that whoever is NOT First Player chooses first.

R: The line in there should be “After deployment” not before.

Q: In Feast for Crows, can freedmen end the Round out of range to kill themselves to create a corpse pile?

R: Yes, they are Infantry (as noted by their type-icon), and the Feast for Crows rules state any time an infantry is destroyed, it can generate a Corpse Pile.

Q: In the game mode Winds of Winter, when do you select your hand of mission cards (before terrain placement, before deployment, before play one is determined)and also are these selected in secret?

R: The hand is constructed once the game mode is chosen, but before any additional steps are taken.
Your hand of selected cards is not open knowledge to your opponent.

Q: Player A has claimed a zone on the NCU board, Player B has not. Mission 4 is in reserve (“If your opponent claims a zone on the tactics board before you do, score 1 VP”) > Player B activates a NCU and decides to swap in Mission 4. What happens ?

R: Mission Cards can’t retro-actively be completed, but if the condition is met moving forward they will be activated/scored.
In this case the card was moved from Reserve to being an Active Mission. The next time its conditions are met, it will trigger.

Q: Can this mission also be scored multiple times in one round? IE. if Player A claims 2 NCU zones before player B claims any does player B score 2 points?

R: Yes  (Not an official answer)