Can Bedeviling Stranger played from hidden target another battlefield? And other question: does it target a unit?

No — when Bedeviling Stranger is played from hidden, the battlefield it chooses must be the battlefield it was played to (Rule 811.1.d.3), so it can't reach across to another battlefield.

And yes, it does target a unit: "return an enemy unit there" makes you choose the enemy unit, so that unit is a target — but it must be an enemy unit at the battlefield you chose, which is the one it was played to. So if that battlefield has 4 or more units, you can bounce one enemy unit there; otherwise the conditional part simply doesn't happen and the unit just arrives.

Think of it as "an enemy unit here" once it comes out of hidden.