"I myself will be leaving at nine."

Translation: Bidh mi fhìn a' falbh aig naoi uairean.

[JouBA51]

would aig naoi uaireanan be wrong? or is it just because it is not in the original?

[Rona460381]

I'm sure uaireannan in 'sometimes' whereas uairean is hours (plural of uair), so uaireannan would be wrong here

[Coiseam]

so, is 'Fhìn' just another version of Fhèin?
would we use it differently?

[Arwasaynemane]

The variant fhìn is used in connection with the first person pronouns, e.g. mi fhìn, sinn fhìn, orm fhìn. The other persons get fhèin, cf. eadar mi fhìn 's tu fhèin "between me and you". In some dialects fhèin is used for all persons.