Synthesized answer
The provided passages do not contain information about what questions remain unanswered in Alexander von Humboldt's "Kosmos: Entwurf einer physischen Weltbeschreibung." Therefore, I cannot answer your question.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Kosmos, Entwurf einer physischen Weltbeschreibung, von Alexander von Humboldt by Alexander von Humboldt