1. Word by word the translation for "¿dónde está todo el mundo?" would be "where is all the world?". Nevertheless, what that sentence means is "where is everyone?" because sometimes in informal contexts we refer to people (in general) as "el mundo".
2. "¿Dónde está todo?" is not incorrect but it means something different. That sentence means "where is everything?". If you want to refer to people it would be "¿Dónde estáN todoS?" because you are referring to many, so it would have to be in plural, both the noun and the verb.
3. Dónde (with accent) is for asking questions or for exclamations and it means "where" . For example: ¿Dónde vives? or ¡Dónde estará mi gato!
Donde (without accent) responds to the "dónde" questions. For example: Vivo donde vive mi mamá. or El gato está donde te dije.
Hope this is helpful!