I don’t think any of the souls that Frostmourne claimed are in the Shadowlands. I think Frostmourne holds onto them and they don’t get to ascend.
I think Uther was an exception simply because of his hard devotion to the Light. I think all the souls inside Frostmourne are still whole. They just haven’t ascended yet.
Uther was tugged on by two powers. Frostmourne and the Light, and therefore the Light essentially tore him in half when it tried to save as much as it could.
Other devoted Light users aren’t necessarily killed by Frostmourne and therefore don’t have their souls tugged on by two different forces.
Likewise, other beings killed by Frostmourne might not have been so devoted to the Light as Uther, that the Light felt they were worth the effort of claiming and tearing in half.
I think Uther’s wound was just his own. I don’t think there are other aspirants running around with Frostmourne wounds, because they’re not in the Shadowlands yet. (Until the blade was broken and they were set free. But still whole.)
But I do find it a little silly that nobody ever noticed this dudes wound because he had a thin piece of cloth covering it. That’s kind of a weak plot point on Blizz part.