It's unknown how exactly Louisoix overcame his tempering. Perhaps because he's also a primal? Anyway, yeah, Bahamut let go of Nael's soul since she wasn't useful anymore. So a primal can voluntarily relinquish their hold on a person, but there must be a way to bring someone to their senses even if that's not the case... unless, again, Louisoix's nature as a fellow primal was what made it possible.

Primals are entropy engines, more or less; their purpose is to distort the flow of aether and create instabilities (not to mention the havoc they wreak on mortal civilizations), bringing the chaos that will return Zodiark. We don't know if the aether primals consume can be replenished, but we do know they have detrimental effects on the landscape around them; the Sagolii desert, for instance, is hinted to be a consequence of Ifrit's summons during the first SMN quest. If the Garlean theory on the Burn being a consequence of repeated summons is true... yeah.

Tempering can produce physical changes, but for the most part it's just warping souls. The only people shown to have biological differences as a result of primal influence are Nael (in her Coil deus Darnus form) and the overdrowned pirates in Sastasha HM.