He is more substantial than most ghosts.
So he's a strong ghost.
He stayed behind with a special mission to protect his family. We know from Monty that is one one of the reasons Shades stay behind for a while, they have something to do.
We also know that ghosts can be formed because the person who died was focussing on a specific mission, and that the resultant ghost is therefore tied to that mission--it's what Harry did in Grave Peril.
Lea has noted him as special.
And Jim has noted that ghosts that evolve and gain new power sources are special.
The way how Uriel interacts with him as an individual worth saving not as some magical appearance.
Well, if Uriel didn't interact with him as an individual, Sir Stuart's ghost would hardly agree to work with him, would he?
Also, we've never seen Uriel interact with any being without a soul to my knowledge (except Sir Stuart's ghost, which, due to the current debate, is not evidence on this point) so we have no basis for comparison. For all we know, he treats everyone like that.
The whole tone of the book makes me think that Morty actually underestimates the number of spirits that have actually souls. But in the end it is not about one or two sentences.
Tone's pretty subjective. I certainly never got that impression.
It is about reading ghost story as a whole and the impression you get from Sir Stuart. He is simply too substantial.
The impression I got was that Sir Stuart was a particularly strong and substantial ghost who worked with an ectomancer, and possibly a line of ectomancers.
It is how the story is build up. We are shown the complexity of sir Stuart without explicitly stating it and Uriel's interest is the confirmation of our suspicions.
Of your suspicions, please. I never had such suspicions, and I certainly don't view them as confirmed.
Ghost story is a show not tell book.
And as I understand it, it showed Sir Stuart as a ghost that, while strong and substantial, was nevertheless a normal ghost.
If he'd done things like go through a church threshold, or disagreed with Mortimer when Mortimer told Harry that he wasn't the real Harry, saying something to the effect of "no, sometimes these ghosts we get from Karrin's dad actually have souls attached" it would be one thing, but he doesn't do any of these things.