If I put Empoleon onto my Bench using its "Emergency Surfacing" Ability, does Torterra's "Evopress" attack add 50 to its damage for the unevolved Empoleon?
Yes, even though Empoleon doesn't have any lower stages attached to it, it is still considered an Evolution Pokemon.
Source: Brilliant Stars FAQ; TPCi Rules Team (2022-02-24)