I had "worst in the world" but changed it to "worst of the world" because duo is often unreasonably picky about using the exact same article in translation. Sometimes you just can't please that finicky green bird.
I'm not sure why no one else has noted this during the last several months, but apparently there is a rule (at least a rule-of-thumb) that with a superlative (e.g., "el/la peor," "el/la mejor," it is always proper to use "del" or "de la." "De" should be used whether the appropriate English preposition is "in," "at" or "of."
That doesn't sound right in English. "The worst in the world" is more typical
Perhaps the Duo programmer is teaching us an expression used in real Spanish. In Spain, I often saw advertisements like "The best bank account en todo el mundo" -- which always gave me a smile. My impression is that this is sort of idiomatic, an intentional exaggeration to emphasize your opinion.
I think you are right. It is my impression that if a lot of people came to a party, someone might say, “Todo el mundo está aquí!”
No. The sentence never says anything about a subject. It is a sentence fragment, really.
no because it's el, not él (the, not he). Accents can be tricky in Spanish.