We're probably clear of needing the spoilers in responding.

The apparatus they left at the end of the storyline does effectively act as a mirror, reflecting the energy form prayers to them back out at the world, so there's not really a gap in that part of things.

As for 'if you could summon a primal out of them'... well, yeah. Remember, we've fought primals of a historical king, an animist faith's totemic animal,and an impossible machine that a grouop was convinced could work, and that was all in one expansion. Absolutely you could summon a primal of one if so inclined. In fact, it already happened; Ramuh is just a local interpretation of Rhalgr, as confirmed by Rhalgr himself.

It's actually never been 100% stated or clear what Louisoix did with the Twelve to try to stop Bahamut; the assumption of 'tried to summon the Twelve, failed' is a commonly-held one, but not actually confirmed, or even to the best of my knowledge stated at any point in-game.