Without more information, the best we can do is speculate.

Thordan I is, at the very least, considered the benevolent progenitor of Ishgard before we exposed the lie. During that time people fought in his name, despite praying to Halone for deliverance. It's not impossible to think many prayed to Thordan himself, if covertly.

Halone is also suggested to be a genuine goddess (maybe), not a primal. Of course there's always "summoning Halone as a primal," but that wouldn't be the "real" Halone. Primals also have their own will and agenda unless summoned into a(n Echo-possessing) mortal vessel, so a "primal Halone" wouldn't necessarily behave in a predictable or controllable manner. Summoning Thordan and his Knights Twelve as primals made the Archbishop and the Heavens' Ward into realized, ideal images of famous historical figures - the heroes of legend come back to end what they began. It's a great PR move to those ignorant to the truth.

Directly answering the questions:

1. King Thordan was probably the strongest primal we've faced to date, somehow using the backlog of prayer from the Dragonsong War and Nidhogg's right eye for sustenance.

2. It depends? A war goddess, maybe not. If it were a primal, it's just a matter of how much aether and prayer is sustaining her, and how powerful we are.

3. PR (see above). Alternatively, mayhap Thordan VII loves Halone too much to desecrate her with a primal summoned in her image, even knowing primals are just shades of mortal desires made manifest?

4. Given the heavy Ascian involvement in Thordan's plan, anything is possible. Perhaps they taught the Heavens' Ward to believe in themselves enough to manifest a primal in the image of one of the Knights Twelve, and the entire lot could only manifest themselves with Nidhogg's eye providing the aether?

5. Maybe? Considering what he knows about primals, it's probable he just selected someone (or something) to summon that's good for PR (see above).

6. He does ramble at length about becoming a god-king... perhaps, but I don't know.

7. Not always Allagans. Sometimes it's the Ascians.