Sniffers can sense other properties about a soul than its strength, yes... eh, so beyond being able to sense the Fixer's state of mind (which he has been doing until now as well) he'd also be able to sense that he is not a necromancer, and that his natural elemental affinity is with air. And the fact that Immanuel was not a Sniffer was part of why Gerald was so surprised when he could not sense him, indeed, although there is more to it than that; while even necromancers aren't able to get any kind of accurate readings on Sniffers - as in, they are unable to tell just about anything at all about a Sniffer just by sensing them, not even where they are - they are just barely able to pick up the presence of Sniffers... that is, they are able to tell that a Sniffer is nearby, but nothing beyond that.

And ASTA's post... technically I didn't miss it, but I did delay reading it because I didn't have time when I saw it, and then forgot all about it. Heh. This is becoming a bad habit... I have to stop getting distracted so easily.
Anyway, I'd everything sounds pretty acceptable thus far; the way the Echoers' powers work and affect them sounds a lot like the relationship between first-generation demonspawn and their powers in that using their abilities grows their power, but also activates their demonic blood and corrupts them. If your explanation ended up being something along the lines of Claw's power somehow causing a degeneration of his Seeds of Good and Evil (which would coincidentally prevent him from entering the common afterlives of The Prophecy, since the growth of the Seeds are what determines the afterlife earned), then eating the hearts of other creatures the same as Harvesters do would potentially counteract such damage... though chances are that if Claw really has the ability to absorb others' Seeds, he would also inherit at least some of the growth of that heart; in other words if he devoured the heart of an extremely good or evil being, the balance between his own Seeds might tip one way or the other, affecting his own afterlife (though I imagine that even if he absorbed a well-grown Seed of Good, the very act of doing so would probably be considered sufficiently evil to counterbalance it).
His power itself should probably still come from his magical energy, though, even if its use affects his Seeds somehow. I also think that the damage to his Seeds should be explained as a result of him using magic at all rather than using this particular kind of magic... as a kind of, I don't know, spiritual disease or racial deficiency. How does that sound?