Can I use Umbreon-GX's "Dark Call GX" attack to discard 2 Double Colorless Energy?
Yes, you can select 1 Energy of each Double Colorless Energy to discard both of them. This applies for discarding your own Energy for an attack and for retreating as well. So if you used Hoopa-EX's "Hyperspace Fury" attack or paid its Retreat Cost of 2, discarding 2 Double Colorless Energy would be legal options.
Source: TPCi Rules Team (2017-01-26), TPCi Rules Team (2017-06-27)
Does Umbreon GX's "Dark Call GX" attack have to discard 2 energy from a single Pokemon, or can I discard 1 energy from two separate Pokemon? Or does it mean to discard 2 energy from each of my opponent's Pokemon??
The term "Pokemon" in the attack is plural, but it doesn't mean all of your opponent's Pokemon. You may discard 2 energy from one Pokemon, or you may discard 1 energy each from two Pokemon.
Source: TPCi Rules Team (2017-01-26), TPCi Rules Team (2017-06-27)
Say my opponent used Turtonator GX's "Shell Trap" attack last turn, then on my turn I attack it with Shaymin EX's "Sky Return" attack. What happens to the 8 damage counters effect from "Shell Trap"?
Sky Return puts Shaymin EX into the hand as part of its attack before the damage counters are placed for Shell Trap's effect.
Source: TPCi Rules Team (2017-06-08)
Say my opponent used Turtonator GX's "Shell Trap" attack last turn, then on my turn I attack it with Tapu Bulu GX's "Tapu Wilderness GX" attack. Does "Shell Trap" place its damage counters before or after the healing part of "Tapu Wilderness GX"?
Tapu Wilderness GX heals Tapu Bulu GX as part of its attack before the damage counters are placed for Shell Trap's effect. So in the end Tapu Bulu GX will have 8 damage counters on it.
Source: TPCi Rules Team (2017-06-08)
Does Machoke's "Daunting Pose" Ability prevent my opponent's Decidueye GX's "Feather Arrow" Ability from placing damage counters on my benched Pokemon?
Yes it does. Decidueye GX can still use the ability against one of your benched Pokemon, but placing any damage counters would be blocked.
Source: TPCi Rules Team (2017-06-01)
My opponent and I both have Sudowoodo with "Roadblock"Ability in play. I use Hex Maniac on my turn to shut off "Roadblock", and then I play 5 Pokemon on my Bench. My opponent plays 5 Pokemon on their Bench during their turn. At the end of my opponent's turn, Hex Maniac's effect ends, so "Roadblock" activates again. Who discards their Pokemon first? If the first player discards their Sudowoodo, would the other person need to discard a Benched Pokemon?
It's the opponent's turn, so their Roadblock goes into effect first. You would discard your Pokemon first, and then your opponent would discard theirs. Even if you chose to discard your Sudowoodo, your opponent would have to discard down to 4 Benched Pokémon because the Roadblock effects happen simultaneously.
Source: TPCi Rules Team (2017-05-25)
When using Rough Seas, can I choose to heal only some of my {W} and {L} Pokemon or do I have to heal all of them?
If you choose to use Rough Seas you must heal all {W} or {L} Pokemon with damage counters on them.
Source: TPCi Rules Team (2017-05-18)
When using Potion, can I choose to heal less than 30 damage from my Pokemon?
If the Pokemon has 30 or more damage on it you must heal 30 damage, but if it has less than 30 damage you do as much as you can and heal all of it.
Source: TPCi Rules Team (2017-05-18)
If Genesect EX uses "G Booster" to attack a Pokemon with Grass Weakness but that Pokemon has a Weakness Policy Tool attached, is Weakness applied or not?
G Booster isn't affected by "any effects on the Defending Pokemon", but the term "any effects" does not include effects that alter Weakness or Resistance. So in this case, Weakness is NOT applied.
Source: TPCi Rules Team (2015-02-26), TPCi Rules Changes Announcement (2017-05-03), TPCi Rules Changes Announcement (2017-05-18)