I'm pretty sure that none of the Inchoroi themselves were soulless husks, just the Sranc and other bio-weapon races.
With their massively superior tech, it's not surprising that Sil went for open battle, although I can imagine what he was thinking at the end......"Oh, well, just another of those battles, our plasma cannons against spear-chucking primitives, the masturbation will be fun.....AAAAAAAGGHH, THE PRIMITIVES ARE SHOOTING ENERGY BLASTS FROM THEIR EYES, FUCK FUCK FUCK"
Yeah, these are pretty much my exact thoughts on it. I think they knew only the most vague details regarding Earwa, what it was like, and what was available there. The whole notion of the Inchoroi even losing these wars
at all sort of resides on their ignorance to magic. Upon arriving on Earwa, at first they'd just see pseudo-Antiquity-level cultures, at least so far as technology goes (the Nonmen-- humans were still all hunter-gatherers at this time I believe, aside from the Cunoroi slaves). Which, to the Inchoroi, would have been all that they considered in the very beginning. Ignorance makes more sense than a reckless rush into battle,
unless they thought the battle would be an absolute cakewalk. In this case, I could see Sil wanting to take the field in order to be among the first -- if not
the first -- of the Inchoroi to step upon Earwa (or in Sil's case, fly down on a freakin' dragon).
As for the concept of souls, I think all of the Inchoroi had them. It seems necessary for the functional plot dynamic of their damnation and motivation in general. The "grafts" are described specifically as being species-wide. Additionally, I don't think they had weapon races before invading (aside from dragons, maybe), as they probably had no use for them before. Once they were on stranded on Earwa though, with their populations radically reduced both from the crash and from the wars with the Nonmen, they realized they needed to cultivate a new army, one that grew quickly, efficient;y, and was easy to manage. Thus, we have sranc. I think a bigger question for me personally is why they used the Nonmen's bios instead of just copying their own DNA. I think there must be a reason for it beyond mocking the Nonmen (and, of course, serving as another Tolkien analogue). They might have believed that using their own "bios" to create soulless husk creatures was taboo, even for them.
The grafting of the onta is something I've wondered a lot about. What the hell did they graft? They used the Tekne to do it, so presumably there is some physical quality that Nonmen and humans possess which allows them to perform sorcery. I think perhaps it has to do with the heart. As aliens, the Inchoroi may not have had a direct equivalent, and even if they did it might not have been enough to use magic (for them, a heart would have just been another organ). So maybe the survivors of the graft were literally giving themselves a Nonman/human heart. Just a thought. I really can't think of any other logical thing that could be physically grafted for the allowance of sorcery, aside from like, adding some portion to their brains or something (I don't see this being the case though).