It's a good point that Sundered Ascians are also shown as being Coocoo for Zodiark - but it's worth pointing out that people are quite able to become cult-minded without being literally mind-controlled into being so. Perhaps upon being reminded of their original selves by the memory crystals, they were so overwhelmed with awe by memories of Zodiark's amazing power and greatness that they voluntarily began to worship him - no tempering required!
As I see it, the memory crystals work a lot like the Starfall Vision, which stir feelings of loss even in folks who remember nothing of the End of Days. Those feelings are part of the soul. The crystals are not sufficient on their own to rebuild the original Ancient, but they are sufficient to jog the memories stored within the soul fragment - and THOSE are the true memories of the original. It's not so much that the Unsundered were creating a facsimile of their original comrade as they were reminding them of what they used to be like in order to stir the true memories. Remember the time...? You were such a joker... You were truly devoted... so on and so forth, everything the Unsundered knew of the person, much as a person would try to help an amnesiac remember who they are.
I see a couple of possibilities here as to why Zodiark didn't Temper everyone. First, it's possible that the Convocation was Tempered as a direct result of Zodiark's Summoning. It was involuntary on Zodiark's part, and he saw no reason to Temper anyone else. There's some evidence of this with Ramuh - he is never shown to Temper anyone, is never stated to have Tempered anyone, and apparently has no desire to Temper anyone, and yet several Sylphs are Tempered. Even keeping in mind that the Tempered can Temper others WITHOUT the presence of the Primal (Leviathan's followers do it, and Ramuh's followers kidnap Sylph seed pods to do it, as well), there still had to have been some original Tempered Sylphs - and they were probably the ones present at Ramuh's summoning.
Another possibility is that Zodiark WAS working to Temper everyone, but powerful as he was he wasn't able to just snap his fingers and do so. He had to find them and Temper them, and we don't know what limits he might have had on his ability to do so. The Dissenters were one group that managed to evade his reach long enough to summon Hydaelyn. Tempering might have even been a design choice baked in from the start - Zodiark was GIVEN the ability to Temper in order to control Ancients and stop their uncontrolled Creation that was causing so much trouble at the time. Zodiark, though, needed to consciously apply this Tempering, and for whatever reasons did not always choose to Temper.
Still another possibility is that while Zodiark was able to overwhelm the wills of many or most Ancients, there were some few with will strong enough to resist even him. Zenos believed that his mastery over Shinryu was possible because his will was greater than Shinryu's. (Whether this was actually true, or whether it was due to his Resonance is a matter of debate.) It just happens that every single member of the Convocation did NOT have sufficiently strong will - and, while Emet-Selch clearly believes no one possibly could, it doesn't mean he was RIGHT.