If Reshiram & Charizard GX has the extra 3 Fire Energy for "Double Blaze GX's" bonus effect and attacks a Whimsicott GX with "Fluffy Cotton" Ability, does the Whimsicott GX get to flip for "Fluffy Cotton" or does the "Double Blaze GX" attack go through automatically?
Technically speaking you still flip for Fluffy Cotton, but regardless of the flip's outcome the Double Blaze GX attack ignores the effect and does the damage anyway.
If I have Focus Sash attached to Marshadow & Machamp GX and then use its "Acme of Heroism GX" attack, which one takes precedence if Marshadow & Machamp GX would be KO'd by damage from an opponent's attack?
You would choose, as they have the same effect, you choose the order of resolution.
If I have Celesteela GX with "Force Canceler" Ability active, what happens if my opponent uses Venomoth GX's "Ten-Card Return GX" attack?
The damage portion gets blocked, but your opponent still gets to shuffle their hand into their deck and draw 10 cards because that part is not being done to YOUR Pokemon.
Source:
Unbroken Bonds FAQ (2019-04-19), TPCi Rules Team (2019-05-09)
If I had used Muk & Alolan Muk GX's "Severe Poison" attack that places extra Poison damage counters and in the next turn use Koga's Trap, do they still place the extra Poison damage?
No, they don't. The new, normal Poison Special Condition replaces the old Severe Poison Special Condition.
Source:
Unbroken Bonds FAQ (2019-04-19), TPCi Rules Team (2019-05-02)
If I Switch my Active Pokemon that is affected by Poison from Muk & Alolan Muk GX's "Severe Poison" attack and Dust Island is in play, does my new Active Pokemon take the extra damage counters?
Yes, it does. Dust Island specifies that it is affected by "that Special Condition".
Source:
Unbroken Bonds FAQ (2019-04-19), TPCi Rules Team (2019-05-02)