Only when the caster is below 5th level
The errata for the PHB has clarified the restriction further from the original printing:
To be eligible for Twinned Spell, a spell must be incapable of targeting more than one creature at the spell’s current level.
By default, eldritch blast does not have a range of self and is capable of targeting only one creature. It is thus eligible to be twinned.
However, the spell becomes capable of targeting more creatures once the caster reaches level 5:
The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level. You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.
Thus, once the caster reaches 5th level and above, they can no longer twin eldritch blast.
Note that eldritch blast (and all other similar cantrips) scale with character level not class level as confirmed (unofficially) by Jeremy Crawford on Twitter. (See Do Cantrips use your character level or class level? for more discussion about that)
KISS Principle
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!)
Best Answer
Yes you can affect any creature.
tl;dr You can affect any creature, but the spell does not make the other creature aware of the caster nor can it be expected to have much of an impact on a creature the caster doesn't interact with.
Similarity to dream in wording and affecting other distant creatures.
The friends cantrip is worded similarly to dream in that they target self and affect another distant creature. Friends does not have the restrictions dream does. Using dream on a distant creature is a more familiar use, but it is another example of this sort of effect.
The nature and level of hostility.
A few definitions of hostility[1,2] could apply here to the story:
Both of those make perfect sense for a creature that is aware of the caster's existence. One that was otherwise of a positive disposition towards the caster would not be after the spell ends.
Ineffectual when the creature is unaware of the caster's existence.
The result of the spell in the reaction of the creature depends on the nature of the interaction the caster has with it.
A creature that is utterly unaware of the casters existence and has never had any interaction with the caster would essentially be unaffected in that there would be no reaction from them. A creature with zero interactions with the caster would be expected to take zero actions as retribution.
Predisposed to enmity
A functional effect of using the spell to affect a creature that doesn't know the caster is that the hostility may linger even if they're unaware of it such that they do not like the caster when they meet them for the first time.