Is the Hadith "All people are doomed except for the knowledgeable" authentic, and what are its chain of transmission and full text, and why was the word "al-ʿālimūn" (the knowledgeable) in the nominative case?

This hadith is not authentic. Al-Albani and Al-San'ani described it as fabricated and grammatically incorrect, and that the correct grammatical form should be "al-ʿālameen" (the worlds). Al-Alusi explained this grammatical form as either an emphasis of the pronoun or a substitute for it, citing a poetic verse that was interpreted to convey negation despite being affirmatively phrased. The mentioned word can be similarly interpreted for "al-ʿālāmūn" (the worlds/people of knowledge) to function as a substitute. And Allah knows best.