I'm no expert on the lore of conjuration, but it's my understanding that it involves transporting creatures from the planes of Oblivion and binding them to serve you temporarily. So, Arniel is in Oblivion, then? Does this mean the dwarves are also in Oblivion? Could they theoretically be summoned, like Arniel's shade can be? Or is summoning shades different to summoning daedra?
Maybe the experiment wasn't a success, and something else happened. Perhaps Arniel disappeared, but not in the way the dwemer did - was he killed by wielding Keening, and then svcked into the soul gem?
I'd be interested to know others thoughts on this!