"Jamais il n'avait connu pire situation."
Translation:He had never known a worse situation.
I am a native English speaker, fluently bilingual for decades. My best hunch is pire replaces the need for an article as it is poetic. Other than that, I strongly recommend please typing this sentence on Google adding why is no article needed due to pire? Bonne chance
It accepts "Never had he known". The plus-que-parfait tense refers to something that happened before something else in the past tense. In this case, "Never did he know" and "Never had he known" mean the same thing (before he knew about this bad situation, he never knew a worse one..)
Why is the plural: "Jamais ils n'avaient connus pire situation" wrong? "Il n'avait" and "Ils n'avaient" sound the same.
Worst is the superlative form of worse. There can only be one worst situation. That requires the definite article in English when worst is the object. You could say a worst situation scenario but there the article is referring to scenario not worst situation.
EG: ...experienced the worst situation..... experienced a worse situation.