Can I use Shadow Rider Calyrex VMAX's "Underworld Door" Ability if there are no cards left in my deck to draw from?
Yes, you can. Attaching Psychic Energy is the main effect of the Ability, which means it can be used even if you have 0 cards in your deck.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
If I use Shadow Rider Calyrex VMAX's "Underworld Door" Ability, do I have to draw 2 cards?
Yes, it is not optional (it does not say "you may"). You must draw 2 cards if you are able to; you can only draw a single card if it's the last card in your deck.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
Does Passimian's "Throwing Coach" Ability work if my Active Pokemon's attack doesn't do any damage to the opponent's Bench?
No, Throwing Coach only adds +30 more damage when you actually do damage to your opponent's Benched Pokemon. In other words, you have to do some bench damage in order to have "+30 more" done.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
Can I use Galarian Slowking's "Mysterious Potion" Ability on a Pokemon that isn't damaged?
Yes, you could try to damage your own Pokemon if you really wanted to.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
Can I choose to search my deck for zero cards if Gengar with the "Last Gift" Ability is Knocked Out (for example, if I don't want to empty my deck)?
Since the search is not optional (it does not say "you may") you must take at least one card if you are able to.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
If Path to the Peak is in play, can I use Liepard V's "Hidden Claw" Ability when I play it from my hand onto my Bench?
No, Liepard V is considered in play by the time Hidden Claw activates, so it won't work due to Path to the Peak.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
If Kecleon has no Energy attached and I attach a Psychic Energy to it when Old Cemetery is in play, will Kecleon's "Chromashift" Ability kick in before checking for Old Cemetery or will Kecleon get 2 damage counters?
Old Cemetery's effect will happen first, so Kecleon will receive 2 damage counters.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
What happens if both Corviknight VMAX with "Lustrous Body" and Galarian Weezing with "Neutralizing Gas" are in play? Do they cancel each other out or what?
Lustrous Body takes precedence over Neutralizing Gas. It doesn't matter which one comes into play first.
Source: TPCi Rules Team (2021-04-08)
Say Team Magma's Secret Base and Mimikyu with "Shadow Box" Ability are both in play. If I put Dedenne GX on the Bench, can I use its "Dedechange" Ability or will it be blocked because it has damage counters on it?
Since Dedechange and Team Magma's Secret Base are simultaneous effects, it is up to the player to choose the order in which they will be applied.
Source: TPCi Rules Team (2021-04-08)
Can I use Houndoom's "Single Strike Roar" Ability if all four Single Strike Energy cards are in play or in the discard pile?
No you cannot. Since the location of all four Single Strike Energy cards are known, there's no way Single Strike Roar would be able to find one during a deck search.
Source: TPCi Rules Team (2021-03-25)