The benched Umbreon's Poke-POWER could only be used if it was brought into the active position. The POWER is self-referencing; it does not mean that as long as an Umbreon is active, you can use it. The specific Umbreon whose POWER is being used must be active. Think of the POWER text as being written in the following manner:
"Once during your turn (before your attack), if this Pokemon is your Active Pokemon[...]"