I will help out here.
The problem as I see it, is that this discussion has become one of trying to contain concepts that are inherently messy. Trying to define whether someone died or did not is a tricky enough proposition, let alone the mechanics of the afterlife. We are approaching a spiritual discussion, on metaphysics and abstract ideas, in a scientific way. Therein lies the problem.
If it helps, think of this a little more like quantum mechanics. Harry both WAS and WASN'T dead. He both WAS and WASN'T a ghost. It's all a matter of perspective. The answer is not quantitative, but qualitative. As Mab says, death is a spectrum not a line. Mortal's fear it and want it to be black, but death is a grey word.
The same applies to the ghost issue. Harry did become a ghost, sort of. The circumstances allowed for his soul, and the ghost, to work in sync. Which I suspect is why he was both as limited as a normal ghost, yet able to do things beyond the ability of a normal ghost. His soul is the x factor. Remember, several major heavyweight supernaturals were working together on this. Not normal circumstances at all.
So morris, I hope that answers your question. He was MORE than a ghost.
As for Sir Stuart, I think the answer is already in front of us. See the quote by nadia.skylark - ghosts that are exceptional can grow and change. Sir Stuart's ghost got a job working in Uriel's house. Sir Stuart was already beyond - wherever the afterlife took him. Uriel wasn't hiring a memory, but a shade of a great man, that had become something more. Consider the power and value of a human soul - especially the creative power. It appears in the books powerful enough to create more, and affect all of reality by creating alternate universes based on it's own choices. Not something even immortals seem to be able to do. Sir Stuart, perhaps by aiding Mort and Harry, and perhaps by attaching itself to new sources of power became greater than the original shade produced. This is why Uriel wanted him.
As for the quote, well done on digging up the original source. I merely took Jim's word for it, and he probably just took someone else's. The spread of misinformation is so easy. So thank you for finding that! But I think it doesn't really matter who said it first, in terms of this discussion. The point is that Uriel said it in the Dresden Files, to Harry. That is basically getting cosmic-level truth.
I personally think that Necromancy uses power from Outside in order to break the rules of reality. Which is why they can do all sorts of things when powered by a human. Hence the tricks of Kemmler and Capiocorpus. But they are very rare, for the most part it seems that beings when they experience irreversible physical death they don't come back. But to say it is impossible...well I think not. As one WOJ states, if you have enough magical power you could rewrite reality. So possible and impossible become somewhat irrelevant.
