Just because it says "prison" doesn't necessarily mean that the the term feels appropriate for the place.
It seems pretty clear to me that even if the place
was used as a prison in the past, that wasn't the reason why Noct awakened there.
Noct was definitely taken body and soul into the Crystal, for what it's worth. The question is how his body ended up on the other side of the world by the time he woke up. I didn't get the impression that Ardyn fished him out and stuck him on Angelgard, so it seems fair to assume that the Crystal itself put him there somehow.
If the Crystal had done the same thing to Ardyn as it had Noct, it's entirely plausible that Ignis could have found out about it in his research. On the other hand, I see no reason why Ardyn would have had access to the Lucis Caelum family yacht, especially since the bros almost certainly needed to use it to get back to Lucis.
But, yeah, Umbra could have found Noct regardless of whether the bros knew where he was. XD;
It'd be pretty weird for the Starscourge to have originated from a place with a tiny portion of Meteor very far from the main body of the Meteor itself, wouldn't it? It's not like Angelgard is positioned in a way that it could have plausibly been broken off from the Disc of Cauthess area of Duscae. That tiny bit of Meteor could be the source of the Scourge, of course, but if it was, it seems more like it'd have been moved from its original location to a new chunk of land rather than having the land moved under it. Then again, Angelgard definitely gives the impression of Astral-modified land given how fantastic its structures appear.
Yeah, this sort of speculation is fun to engage in regardless of whether it ends up being confirmed or not. ^_^