Treat Friends as a Buff(self) and the Complication Goes Away
Pursuing the "target" word in the rules is an over complication that misses the point of the spell. While cantrips are not particularly powerful magic, a buff is a buff.
In the case of Friends, the limitation is that you can't BS a group of people but you can BS one person a little better than usual when using a Charisma based Skill. Until you make a Charisma based interaction with that creature, Friends doesn't do anything at all. Friends is a catalyst or enhancement for your subsequent action, or an attempt at Persuasion. It does not create an independent affect on another creature.
Two examples to illustrate the serial nature of a buff:
I cast Bless on our party's Barbarian when we encounter an Ogre. The Bless doesn't influence the Ogre unless the Barbarian does something to the Ogre while under the Bless buff. If he makes an attack on the Ogre, then the Buff can have an impact on the Ogre who now has an increased chance to be hit/hurt. If I bless the Barbarian but he doesn't attack the Ogre, Bless does nothing to the Ogre. The Barbarian has to Choose to attack the Ogre for the buff to matter. (Just as with friends you have to Choose to activate the affect of the cantrip).
My wife goes to buy a car. The car salesman does his best to influence/persuade her via the usual sales tactics. (Friends cantrip analogy). He's got more chance of success if she's alone. With me or her mother (a shrewd bargainer) sitting beside her the salesman might convince her with his smooth talking ploy - but neither of us since he's directing his charm at my wife. Like the Friends spell, once it's over my wife is likely to be annoyed/pissed at the car salesman for laying all of that charm/ on her after his "spell" wears off.
Some of the higher level influence spells, like suggestion or Charm, also require a serial action be directed at the creature or the Charm itself doesn't really do anything.
Friends limits your ability to be charming/persuasive to one creature. You choose the one to whom you want to be more charming/friendly.
Compare Friends to a Cantrip like Sacred Flame: in that case, the target is directly affected by damage (or save to avoid damage). No second/serial action by the Cleric is required to create the damage affect.
Can this creature be one you can't see, such as one behind a door?
If you have a way to interact with that creature behind the door, a way that takes advantage of your Charisma, of course.
Consider this: a blind man can't see you, but he can interact with you, and perhaps try to Persuade you to do something. (He may have advantage in doing so when you are both in complete darkness!)