Also, he might well be a Prince. After all, they haven't all existed since the beginning. Meridia became one, as did Malacath. Sheogorath wasn't there from the very beginning either.
One does not become a Deadric Prince because one has an extreme amount of power. One becomes a Daedric Prince because they are the embodiment of some concept - the first Princes, like Azura, Nocturnal, and Hermaeus Mora came into existence because they were the embodiment of Time/Beauty, Pure Darkness, and Miscellaneous Ideas (respectively, and only very generally speaking). Some of the others, like Malacath, Meridia, and Sheogorath only came into existence when Nirn was created, as their spheres relate only to mortal concepts - The Spurned/Shunned, Finite Life, and Madness/Unpredictability (respectively). All of the Daedric Princes existed by the time that Nirn was in existence for a reason - they are all associated with Nirn in some way (whether they like it or not).
So, just because Umbra kidnapped a portion of Clavicus Vile's realm (it still being his realm currently) and a portion of Vile's essence, doesn't give him any sort of Daedric-Prince-level of authority.
Note that Jyggalag existed before Nirn was created, and that his transformation into Sheogorath coincided with Nirn's creation - Predictability/Order before Nirn, Unpredictability/Madness after.
EDIT: Umbra isn't even a Daedroth of some kind. All that we know of him is that he's a powerful soul (hence why the sword was so b.a.). So to say that Umbra is a powerful Daedra Lord is still a mislabel - he's just a powerful Something Lord until we find out more....