Can I play Plus Power or Electropower or Power Tablet on my first turn if I'm going first and am not allowed to attack?
Yes you can play those. Keep in mind there are some Pokemon (like Latios-EX with "Fast Raid" attack) that are allowed to attack on first turn.
Source: TPCi Rules Team (2021-11-11)
If I have Hiding Energy or Snow Leaf Badge attached to my Active Pokemon, would my opponent's Leafeon VMAX's "Grass Knot" attack do any damage to it or not? If yes, would "Phoebe" remove the effect of Hiding Energy or Snow Leaf Badge for the damage calculation of "Grass Knot"?
The effects of Hiding Energy and Snow Leaf Badge are not blocking any damage from the attacking Pokemon, they are changing the stats of the Pokemon they are attached to. So they are not really affecting the damage done by Grass Knot; it is still 60x the Retreat Cost, but the Retreat Cost is now effectively zero. And since those cards are not affecting the damage being done (they're only affecting the Retreat Cost), Phoebe will not help it do more damage.
Source: TPCi Rules Team (2021-10-07)
Can I choose one or more parts of Greninja V-UNION with Vaporeon VMAX's "Bubble Pod" attack, and play it on my Bench?
Bubble Pod cannot put Greninja V-UNION into play because Bubble Pod will only get one card of the V-UNION, and one piece of a Pokemon V-UNION cannot be brought into play alone.
Source: TPCi Rules Team (2021-09-30)
Can I use Heavy Ball or Level Ball to search for a Pokemon V-UNION card?
No you can't. Even if a Pokemon V-UNION has a Retreat Cost of 3 or more when it's fully assembled, you still can't search for any of the parts with Heavy Ball. And even though some of the parts don't have any HP listed, you still can't search for them with Level Ball.
Source: TPCi Rules Team (2021-09-30)
Can I use cards that search for a Pokemon, like Ultra Ball or Poke Ball, to get a Pokemon V-UNION card from my deck or discard pile?
Any effect that lets you search your deck or discard pile for "a Pokemon" will let you use that effect on a Pokemon V-UNION card; but remember, you cannot put a single Pokemon V-UNION card into play.
Source: TPCi Rules Team (2021-09-30)
Can I have less than four pieces of a Pokemon V-UNION in my deck? Or can I put multiple copies of the same piece as long as I do not exceed four total cards of the same name?
Yes you may have less than four pieces of a Pokemon V-UNION in your deck, and you can have multiple copies of the same card; although without all four pieces that will make it impossible to get it into play.
Source: TPCi Rules Team (2021-09-30)
Can I play a Pokemon V-UNION as my Active Pokemon or on my Bench during setup?
A Pokemon V-UNION is not considered a Basic Pokemon, so you can't put it out while setting up to play.
Source: TPCi Rules Team (2021-09-30)
Do Pokemon V-UNION count as one Pokemon or four for purposes of bench space or other effects?
While in play a Pokemon V-UNION is made up of four Pokemon cards, but it counts as a single Pokemon occupying only one bench space or the Active Spot.
Source: TPCi Rules Team (2021-09-30)