Can someone clarify this "se stessa" thing for me? What exactly does it mean? When should it be used? Are there masculine and plural versions? And some other places indicate that it should be "sé" not "se". Is that true?
"Sé" means "self" in the third person, and "stessa" adds emphasis. Something like "her very self". "himself" is "se stesso", themselves, "se stessi" or "se stesse". "Sé" is usually written with the accent mark, but in this expression it is not. It's used, as here, when someone acts on him- or herself. You can also say "me stesso", etc.
Another way of considering "stesso/a" is "the same" e.g. la stessa cosa" = "the same thing" One of those flexible parole italiani
I feel that "nello specchio" could also be good, but I'm not sure if maybe it's only colloquial form.
could you say 'sullo specchio' like you do in spanish although in spanish it is 'en el espejo' and that is the same as 'in'
Would "at the mirror" be a correct translation intead of "in the miror"?
I think in this case it's "at" because you're not literally "in" the mirror.