In prescriptive grammar, the subject complement, if a pronoun, is supposed to take the subject form rather than the object form. The editors for the 1920 LDS edition apparently recognized the so-called ungrammaticality of “ it shall be us” but probably found the purportedly correct “it shall be we” sufficiently unnatural that it was decided to emend the us to the reflexive pronoun ourselves (which does not distinguish between subject and object forms).
There are only two examples in the original text of a reflexive pronominal subject acting as a subject complement:
Both of these examples differ from Alma 46:27 in that they involve the expression “save it were X”, where the it is an expletive that fills the subject position. Nonetheless, if a nonreflexive pronoun were to be used in these two examples, prescriptive grammar would require the subject forms rather than the object forms, especially in formal language (thus “save it were he” in preference to “save it were him”).
The critical text will restore the original object form us in Alma 46:27. For further discussion of examples like this, see under 2 Nephi 1:27 (which deals with the reading “it was not him” versus “it was not he”). Also see the general discussion under subject complement in volume 3.
Summary: Restore in Alma 46:27 the original object form us for the subject complement in “it shall be us”; as a grammatical emendation, the reflexive ourselves sounds better (“it shall be ourselves”) than the subject form we (“it shall be we”).