You can't play a Trainer-Item card for no effect. (This cost of playing a card is not part of the effect.) Playing Rare Candy with no Stage 2 in hand would have no effect, and thus is not allowed.
If there is a valid target, attacks can be used regardless of whether there is a net effect or not. The restriction on attacks is generally that they can't be used when there is no valid target. For example, Eelektross' Slurp Shakedown can't be used when your opponent has no benched Pokemon.
However, Terrakion EX's Pump-up Smash can be used even if the attack would do no damage and you have no energy cards in your hand, assuming there is a valid target (which there always will be).
Usually, i would agree with your first statement, but i dont think its true. If you read the text on Rare Candy, its quite interesting. "Choose 1 of your Basic Pokemon in play." This is the ONLY requirement to the card, outside of its new first turn and new basic rulings. I can chose Carnivine on my bench as a Basic Pokemon and fufil everything Rare Candy forces em to do if he has been there a turn. The very next line starts "
If you have a Satge 2 in your hand that evolves from that Pokemon, put that card on the Basic Pokemon." This s a conditional effect is it not? Choosing a Basic Pokemon is a preformable action, even if it doesn't acomplish anything.
Consider how coin flip cards work, i'll choose Recycle in this case. "Flip a coin." Again, the only part of the card that is mandatory If you play Recycle you cannot not flip a coin. "If heads, put a card from your discard pile on the top of your deck". It says nothing about what happens if its tails, because cards now are assumed that if the result intended is not achieved, the card fails. How is that different from rare Candy. You pick a target, find you have no target and the card fails and is discarded, just like a Tails flip on Recycle.
Also, looking at cards you can't play to no effect, like Max Potion. Very, very different text. "Heal all damage from one of your Pokemon." Already, it is starting with a much more direct conditional statement. It is telling you you cannot select a Pokemon that has no damage to heal, because its all in the requirement. "Then, discard all Energy attached tot hat Pokemon." Again, there is no if statement to back that it could be played to no effect. Everything on the card is instructional, leaving no room for playing the card as an empty effect.
I mean, Terrakion ex doesnt even give you leeway. It tells you to attach those darn energy! And we're saying you can hold them against what the card says with it being a private zone. Rare Candy shouldnt even needa privatezone, but with it, has a much stronger case than Terrakion does.