Metropolis/WMG

The whole film is Maria's hallucination
Being the sole person responsible for leading La Résistance and keeping it peaceful has to put pressure on Maria, and so she spends months or years waiting for "the mediator" she hopes will take the burden of responsibility from her. However, just before the "events" of the film take place, something brings her to the devastating realization that the mediator will never materialize.

This realization causes Maria to have a complete mental breakdown. Rather than come to grips with reality, she projects her hopes onto a random boy she saw one day - Freder - and lapses into a dream world where he is the central character. This explains the overall fantastic and improbable nature of the events that take place. The heavy symbolism present throughout the film is a sign of the way she sees the world in mythical/Biblical terms, like the parables she tells. It also explains Maria's character decay from independent rebel leader to pathetic damsel in distress; she's overwhelmed by the stress of leading alone and wants to surrender that power to someone else. The robot subplot is an expression of her fear of her own sexuality. The simplistic way everything is resolved reflects her naive view of the world in general and labor disputes in particular.
 * This theory is somewhat undermined by the longer restored cut, which adds some complexity to the ending, and also showcases Maria repeatedly risking her life to save others, especially children, even late in the movie.

The Heart Machine is a prison for an Eldritch Abomination.
In ancient times, humans were sacrificed to keep the creature sated, remaining dormant in its lake of blood. In modern times, the temple was rebuilt into a vast machine. When a portion of it exploded, Freder was granted a vision of this era. Now, the Heart Machine pumps blood collected from workers, hospitals, and elsewhere, redirecting it all to the dark god inside it.
 * The Heart Machine is not the Moloch Machine, though that is not made clear until the rebellion at the end. However, the two are probably connected in some way since the Heart Machine is implied to be central to all of the city's machinery.

"Der Schmale" (the thin one)...
...is related to The Slender Man Mythos.
 * Now that you mention it, he does spend most of his time stalking the protagonist and his friends, albeit on Fredersen's orders.