Rules of Primal summoning

[Jandor]

These are both things that I also think are the case. Although yeah, now that you mention it, I can't say I'm exactly sure as to why I think that.

Both just seem kind of obvious I guess?

The Allagan sealing would be virtually pointless if you could summon multiple copies of the same primal, so I'd say it's basically confirmed that you can't.

We saw Ysayle control the Shiva incarnation, and Zenos does mention the echo can be used to force our will on to a primal. Whether we can go as far as to say that people without the echo that self-primal definitely end up self-tempering is rather ambiguous though. The goals of Thordan and Yotsuyu aligned pretty well with their respective primal forms, so it's hard to say exactly how much they were in control or not.
You'd think that'd be something the Scions would have looked into before letting Ryne have a bash at being a primal.