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?
"Il s'en alla" is correct; use a conjugation reference, a conjugation web page for instance; you get all forms and the missing forms are shown. (confirmation)
There is no difference in meaning at all; the only difference is the register. You would use the "passé composé" normally, as the "passé simple" is only literary (rare in the spoken language and everyday written communications).
"Je m'en fus" is an idiomatic form that does not exist in the "présent" nor in the "passé composé". "Je m'en suis été, tu t'en es été,…" are not used.
Here is a link that will provide some precisions: s'en être.
