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."
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.
no because it's el, not él (the, not he). Accents can be tricky in Spanish.
In previous sentence I had "mas difficil" according to Duolingo means most difficult: highest level of difficult. Here we have only peor without mas. Why here in translation is worst (highest level of bad) but not worse?
Not trying to denigrate everything that's great about Dúo. I find prepositions ambiguous on the site. Might be just as easy to write code that's more liberal for "multidefinition" words. I also find prepositions and idiomatic expressions really challenging in general. I've been at it on Dúo for about 7 years. Forgive me for venting on a public forum.