Can I use Inteleon VMAX's "Double Gunner" Ability if my opponent only has 1 Benched Pokemon?
Yes you can, but not if they don't have any Benched Pokemon.

Other known variants



x2Double Gunner
You must discard a
Energy card from your hand in order to use this Ability. Once during your turn, you may choose 2 of your opponent's Benched Pokémon and put 2 damage counters on each of them.

G-Max SpiralYou may put an Energy attached to this Pokémon into your hand. If you do, this attack does 70 more damage.
Can I use Inteleon VMAX's "Double Gunner" Ability if my opponent only has 1 Benched Pokemon?
Yes you can, but not if they don't have any Benched Pokemon.
If my opponent has Duraludon VMAX with the "Skyscraper" Ability in play, when I use Inteleon VMAX's "G-Max Spiral" attack and choose to return the only Special Energy card attached to Inteleon VMAX to my hand, does "Skyscraper" prevent the damage or not?
The choice to remove the Special Energy card is made first, but then damage is done and finally effects of the attack. So removing the Special Energy card is an effect that doesn't happen until damage has been done and therefore blocked by Skyscraper.