If my opponent used Magnemite's "Mirror Shot" attack last turn, and I try to use my GX attack this turn and flip tails, do I lose the ability to use a GX attack for the rest of the game?
No, because since you flipped tails the attack "doesn't happen".
Source: Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)
When I use Arceus & Dialga & Palkia GX's "Altered Creation GX" attack do I take an extra Prize card only if I Knock Out that Active Pokemon with an attack, or do I take an extra Prize for ANY of my opponent's Active Pokemon that get KO'd by my attacks during the rest of the game?
You get to take an extra prize if your Pokemon's attack Knocks Out ANY of your opponent's Active Pokemon during the rest of the game, not just the one that was the Defending Pokemon at the time of the Altered Creation GX attack.
Source: Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)
Do I have to take any cards from my deck if I use Charizard & Braixen GX's "Brilliant Flare" attack?
It says "may", so you can choose to not search your deck; but if you do search you must take at least one card from it.
Source: Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)
If my opponent has used Latios GX's "Clear Vision GX" attack, would my Keldeo GX be able to use its "Resolute Blade GX" attack?
Yes. The effect of Clear Vision GX is placed on each of the opponent's Pokemon, and since Keldeo GX's Pure Heart Ability prevents all effects of attacks done to it by your opponent's Pokemon GX, it is unaffected by Clear Vision GX.
Source: TPCi Rules Team (2019-09-05)
If my opponent used Latios GX's "Clear Vision GX" attack on me, could I use Channeler to remove that effect?
Yes you could, because Channeler removes effects from both you and your Pokemon.
Source: TPCi Rules Team (2019-08-01), TPCi Rules Team (2019-09-05)
If my opponent used Latios GX's "Clear Vision GX" attack on me, could I use Bonnie to allow my Zygarde-GX to use its GX attack?
No, you cannot. Although Bonnie would normally let Zygarde-GX re-use its GX attack, the effect of Clear Vision GX is on Zygarde-GX and that still prevents using *ANY* GX attacks.
Source: TPCi Rules Team (2019-09-05)
If my opponent has used Latios GX's "Clear Vision GX" attack, do I get to draw 7 cards if I use Hala?
If you already used your GX Attack before Clear Vision GX, then yes you get to draw 7 cards for Hala. Otherwise no, getting hit by Clear Vision GX doesn't mean you have used your GX attack for the game.
Source: Unified Minds FAQ (2019-07-19), TPCi Rules Team (2019-08-01), TPCi Rules Team (2019-08-29)
Does Gengar & Mimikyu GX's "Horror House GX" attack prevent me from using Viridian Forest?
No, it does not. Discarding a card from your hand is not the same as playing a card from your hand; the card goes to the discard pile, not "in play".
Source: TPCi Rules Team (2019-08-22)
If I used Poipole's "Knockout Reviver" attack, but then on my opponent's next turn they play Channeler, would that allow them to take a prize for KO'ing Poipole?
No, it would not. The effect of Knockout Reviver resides on Poipole, and the opponent's Channeler only clears effects on the opponent or their Pokemon.
Source: TPCi Rules Team (2019-08-22)
What happens if I use Steelix's "Gigaton Shake" attack, and then my opponent uses Mawile GX's "Captivating Wink" Ability during their turn? Would those Pokemon I put into play be able to attack on my next turn (since it was not during MY turn)?
No, they would not be able to attack.
Source: Unified Minds FAQ (2019-07-19), TPCi Rules Team (2019-08-01)