1. Can Hidden Enchantments on a Banished creature be revealed even after the creature has been Banished?
It is still in play just not in the Arena so I see no reason why not.
2. If a friendly creature is destroyed in the arena, can Valshalla gain Wrath tokens even while she is Banished? Reversely, if a friendly creature is destroyed while Banished, can Valshalla gain Wrath tokens?
Both would depend on how the creature is destroyed. [mwcard=MW1C39] Valshalla, Lightning Angel[/mwcard] says that a friendly creature has to be attacked and destroyed by an enemy creature. As far as the 2nd part goes at this current time we have no way to attack creatures that are not in the arena. At the bottom of this post I give a long breakdown of what happens when something dies while banished.
3a. If a creature with Eternal Servant is destroyed while Banished, can they be summoned back?
Maybe
Read more about this interaction below question 4.
3b: If they can, will they still Banished until the 3 turns is up, or will they be immediately returned to the arena in the same spot?
Once it hits the discard pile it returns to the same location it was banished from if you pay the cost again.
Read more about this interaction below question 4.
4. If a creature was Banished while it was Stunned, when will the Stun be removed?
After the next time the creature takes an action.Sleep would work the same way once the creature takes damage sleep would turn to daze.
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To understand further we have to first look at -[mwcard=MW1I01] Banish[/mwcard]. - Banish says that the effect from the spell also has the memory of the last zone the creature was in when Banish resolved.
What does it mean to be Destroyed:
When an object is destroyed, everything attached to it is destroyed and discarded. This includes damage, condition markers, tokens, enchantments, and conjurations. Ability markers are returned to the Mage. Then, the object is placed in its owner’s discard pile.
The object may then resolve a “destroyed” effect. For example, a Plague Zombie will explode. Some effects replace being discarded such as Cantrip or Obliterate
Next we have to look at Eternal Servant:
All of the creature’s attacks gain the Piercing +1
Then, should the servant be destroyed, the Necromancer may Reanimate the creature by paying its casting cost. The servant returns to play at the end of the round (see “Reanimate” in codex). You can assign the Eternal Servant marker to it, or hold the marker to place on another undead creature later.
How does Reanimate (Effect) work
This creature reanimates upon death. If this creature is destroyed, it is moved from the discard pile to the zone it was just destroyed in, and placed face down in that zone, with a face-down action marker on it. This creature is considered temporarily out of play. At the end of the round, the creature card is flipped face up and is Summoned into play. Reanimate does not occur if the creature is removed from the game when it is destroyed. See “Obliterate.”
As Far as if the creature can come back or not depends on a few things I noticed. So I sent the info to the rest of the rules team