Can anyone else besides the President appoint an acting Speaker when the Speaker's office is vacant?
No, according to the context provided in 95(1), when the office of Speaker is vacant, the duties of the office shall be performed by the Deputy Speaker or, if the office of Deputy Speaker is also vacant, by such member of the House of the People as the President may appoint for the purpose. Thus, it's strictly the President who can appoint an acting Speaker when the Speaker's office is vacant.