Prepositions

I guess the preposition (one of location! so "pour" wouldn't count anyway) is a good indicator that it is the intransitive version of the verb but the intransitive version of the verb does not necessarily demand a preposition. However the lack of a direct object shows that "retourner" is used intransitively here and therefore has to be conjugated with "être".

"Pour" is optional in this case, as far as I understand it. For the sake of translation, "le voir" is already equivalent to "to see it", but you could add "pour". "Il vient nous sauver" = "il vient pour nous sauver". There may be a slight nuance, but it's essentially the same.

+1