I'll be durned Danny, but most of that is excellent. Really excellent.

I was laying in bed last night re-reading my Bible passages on the Scapegoat when it occurred to me that the demon name for the Giant Ghost might be the Scapeghost. Yeah, it's kind of a sappy name, but the irony would not be lost on either the demon or the spirit of the giant.
In context, its not a bad name at all.
However, because I'm a bit of a cut-up and a jester, my first thought was...sorry to say...
Space Ghost - Wikipedia, the free encyclopedia
Mostly in the context of the fictional talk-show he hosted... You know your players better than I- if you have anybody in the group who is a pop-culture clown, you might want to avoid that name.
But I'd also like the a really fine name for how others would perceive it, who might know nothing of its background but would only know how weird and frightening it seemed to them.
I'm also trying to think of a relic and/or an artifact (it doesn't have to be one from a gaming book, could be from myth, religion, or something you just thought of) that would be a suitable match to the thing that it might have gained control of over time.
Right off the bat, my first instinct would be to go with a Scarab of some kind.
Scarab artifact - Wikipedia, the free encyclopedia
Ancient Egypt: the Mythology - Scarab Beetle
Does it control the swarm (in which case, the swarm's possession of it lets it act freely)? Does it empower the creature?
The one thing I'm not gonna use is the implication you made of it killing and consuming others (if I read you right). I don't want it to be a killing machine, rather I want it to be a frightening, weird-creation machine. That is encountering it will temporarily "mutate or transform" the ones it encounters, in very bad and bizarre ways. But it won't result in immediate physical death so much as psychological and spiritual "staining and corruption." I can though see it inflicting long term diseases of the body, mind, and soul through extended exposure. And causing others to do things against their nature.
Instead of it being a mere killing machine, perhaps it is like the Sin-Eater as depicted in X-Files- a sentient "Death" Tarot card that transforms those it eats. It consumes you- yes, a painful and horrible experience- but it then excretes you as a living and transformed being.
After discovering this- most likely as those thought irrevocably dead start returning to their respective villages- the horror dies down and the creature becomes revered by some as the living incarnation of the will of a god whose portfolio includes the cycle of life and rebirth. Druids, perhaps?
Of course, given its link to the infernal, that might not be the whole truth...perhaps not everyone comes back. Perhaps some of those who return are demon-possessed.
Other than that I like all of your ideas.
Thanks for the links too by the way.
Thanks!