He had a physical message? Proof of his irrelevance and worthlessness, since the Dagoths are in constant communion with the Heart and its keeper. That's the whole point of the operation. And he had low level stats, since you're gettin' jiggy with gameplay excuses and you are sent to kill him at a lowish level.
It was not a message to him, it was for Nerevar. Dagoth Ur wanted Nerevar to stop by Dagoth Gares, and to probably get Corprus, and to get the message. He had a written message from Dagoth Ur to Nerevar, something that no other ash priest in the game has. Dagoth Gares proved that he had powers to inflict Corprus at a whim not once but twice. If every ash priest could do that, then the Temple would be much more worried.
And that wasn't really the Ordinators/Armigers fighting the legions...
They would have ended up doing so. They Ordinator are Indoril, so they were, and are, strongly opposed the Imperial presence. If it came to war, they'd be fighting the Legion.
I don't deny that Akulakhan would have tipped the scales in Dagoth Ur's favor, and that's what he was waiting for, but if he felt he could pull it off without a stompy robot, I doubt he would have waited.