Why can't "sûrement" come at the beginning of the sentence: "Sûrement il reviendra . . . "
You would say Sûrement qu'il reviendra... but that would not be very natural, or at least not as common as Duo's sentence.
Why is "il retournera" incorrect?
You cannot use "retourner de".
To return from Japan = "rentrer du Japon" or "revenir du Japon"
To return to Japan = "retourner au Japon"
Noted with thanks!