It's exactly the direction I think they're taking him in, especially if you attribute Varis's words in the tent to him and not Varis.
I wouldn't necessarily go as far as "tempered" just yet. Whilst it is likely that the Blessing of Light is a form of tempering, from what we've seen of ourselves and the Ascians, there's no indications of loss of free will. It's likelier to be a more advanced manifestation of much the same phenomenon, whereby the subject retains their free will but feels a nudge to answer a higher calling, with the carrot being the substantial power it offers. Maybe the Echo plays that former role and then the Blessing comes as the "reward", with the incidental side-effect of spreading whichever force it's aspected to, i.e. darkness/light.What I struggle desperately with regarding Hydaelyn, Zodiark, and the Blessing being Tempering is the Enigmatic Figure's words: "How many years have I waited for this moment... For one possessed of Her blessing... For you..." (Assuming that is him!) Why would he need to wait and seek out a Tempered follower of Hydaelyn to bring back the Night and stop/reverse the Flood of Light? Did he need a rebellious soul? This explanation feels unlikely—if he knew of the impending apocalypse why would he seek out that which would seem to hasten it? Is it because he arrived on the First after Arbert had left and none remain on the First with the Echo? But he wouldn't have needed to wait so long because... because Minfilia is there... she qualifies on the basic Echo necessary level. The Echo can be replicated via the Resonant too—does that mean an artificial Tempering can be made? Something is missing. Something is missing. Something is missing.
Regarding the Echo being something that could be implanted artificially, it's worth recalling at what costs. It's not easy to do, it required stripping out souls (the aether) from other beings and the host isn't even guaranteed to survive the process. It has all the trappings of Ascian techniques, as presumably even the Allagans did not know how to do this, and neither did the Garleans until Aulus came along (although it is an elaboration on aetherochemistry in a way), and I doubt it is common knowledge.
I too thought it could be parsed in terms of the Echo, which is more flexible in that both champions of darkness and light can bear it, but it's still bizarre phraseology, since the blessing has a specific meaning to it. What he may be referring to is what you have accomplished with that blessing, meaning you could achieve similarly great things with a darkness equivalent... however, that would either require Zodiark or some artificial version of him, which I suppose is what you mean by an artificial tempering. The trailer mentions the hero harnessing light and dark, which may be where this all comes together.
Most likely because this is only the first flood of Light of which we're aware. It's possible that it had occurred on other worlds but that the Ascians drove it back before it could go much farther. It's also possible that there are worlds outside the shards, where light or darkness can exist in a purer form, supporting life forms compatible with each - i.e. more complete versions of the void/what the First is becoming.The Kuribu of Amdapor is most likely based on a Sin Eater, as is their Winged Lion. However, we see animate Kuribu and Lions on the first—though the Kuribu differ slightly (feeling more alive! actual hair, not carved stone, etc!) Did the Amdapori accidentally summon Sin Eaters before? Is the Apocalypse Cow FATE in Eureka Pagos transforming the Holy Cow from dzo to Winged Lion mean that it was a Sin Eater itself? Since the Palace of the Dead features pomanders that turn the player into a succubus and later a very different looking kuribu mean that that it transforms them into a Sin Eater akin to the Voidsent? If Sin Eaters have been appearing for about a century and the Amdapori have depictions of them at least 1600 years old... from whence did they come? I have to image this has more to do with the time between Shards being dissimilar.