The book: the mysteries (revised) came out before realms of power: Magic and so the mystery for spirit familiars seems very much at odds and different than how magical spirits are defined in the later book. Would you need the mystery of spirit familiar to bind a magical spirit as created with realms of power: magic. (they are built very differently in including stats, virtues, flaws, magical traits and such).
I'd generally assume so - the advantages of an immaterial familiar are varied and great.
... and the Spirit Familiar virtue is awesome in itself!
I think so too.
The virtue lets you have a spirit familiar. The virtue defines how these are better than normal familiars, and these benefits are very good for the right character, especially if Might spent to help fatiguing Sta rolls apply to spell casting. (I'd allow it, but am probably a minority.)
The virtue does not explain how to create a familiar of Might X, just as the core rules have rules for normal familiars but no rules for designing them.
RoP:M does have rules for creating animals and spirits with Magic Might. You don't need to use these at all, either for a normal familiar or a spirit familiar, and can just make up some stats and powers, as you like.
I would think that you would need the mystery virtue to get a spirit familiar.
Indeed. Spirit Familiar or Theurgic Spirit Familiar, depends on your cult. You can bind demons with Chthonic Magic, of course, but that's terminally stupid.