I agree about the artifact bit as well. Like they couldn't just come around the other side of the mountains? That one pass could hardly be that critical. Even without the fortress that pass would be near impossible to crack, and if they had the magic to create the Sentinel, why not just cut to the chase and collapse the whole pass if they were that afraid of invasion? Likewise the raw magic to create an artifact level Gauntlet would suggest enough to bore a hole through the mountain around the whole keep, or even just collapse the mountain down on it.
I would tend to take those two being "artifacts" with a grain of salt. They are hardly more powerful than souped up magic items, no greater than lesser artifacts, and likely only as powerful Epic level items, or perhaps just something like the new weapons (items) of legacy.

As for the titles, most people tended not to take such specific titles without good reason. He'd be more likely to call himself a King than a Grand Duke if it came to ego. So it is more likely to be a title derived from someone else, and still more likely that it should plain be redacted to something else.