Why do you keep insisting that "What or whom?" is wrong. It's actually more correct than "What or who?" which you have as your correct answer.
Why should it be more correct? I would use "whom" for komu or koho, but not kdo.
"Kdo" is nominative, so "who" is correct. Absent context, the question could easily be something like "What or who caused the accident?"
Absent context, "What or who(m) seems most likely a response to a comment about what happened to someone or something; i.e., the object rather than the subject of the sentence? I've never heard "What or who?" uttered as a sentence in English!