If I am not mistaken, the fut in the expression
Il s'en fut
is the third person of être in the passé simple and it means "he left".
Apart from a confirmation regarding the above, is this expression the only way to express the equivalent of "il s'en va" in the passé simple? Or one can say "Il s'en alla" as well? Is there any sutle difference in terms of meaning or cases of usage?
What about the opposite? Can one say "Je m'en suis" instead of "Je m'en vais" for example?