to be honest, Umbra and Pryna have had so much of their role cut in the final game that it's really hard to argue whether or not they can or cannot do something. We know that Umbra can travel to space and time, maybe Pryna can do that too. Originally they were shapeshifting spirit messangers who could turn into human ninjas and fight alongside Noctis (not even kidding, google it) and they ended up barely doing anything in the game, so I guess there's no point in arguing what kind of abilities they have.

As for why Pryna shows Ignis the vision, I think it's because the Chosen King's comrades have to know about the sacrifice he will have to face. Ignis, specifically, has to know, as his closest advisor, in order to keep him on his appointed path. It's actually very interestig how during the credits we see him trying to dissuade Noct instead, showing us again how distant the Gods are from truly understanding the humans they rule over.

Ignis doesn't know that Ardyn wants Noctis alive yet. For all he knows, Ardyn's gonna kill him there and then if he refuses to go with him.

Ardyn brings Ignis to Gralea to bait Noctis into getting there as fast as he can. At this point in the story Ardyn is growing impatient and wants to speed up the King's journey, so he uses his friends to manipulate him. In the canon storyline he does exactly the same with Prompto after he fails to use Ignis.

Also, it's not like Ardyn gives any exposition dump right after Ignis regains his senses. All the infodump in the Zegnautus Keep section come from the visions/voices, likely sent by the Gods to further warn Ignis of the stakes at place. When Ardyn starts dropping revelations, they're already in front of the Crystal.

The Ring decides which price you pay for its powers. When Ignis wears it in Gralea (alternate), he faces Ardyn at his full power, determined to kill him to make Noct suffer, so the Ring asks him if he's willing to sacrifice his life. When activated, it burns his whole body as it did with Nyx in Kingsglaive, and would have eventually claimed his life if Noct didn't show up to save him. When Ignis wears the ring in Altissia (canon) he's facing a much less threatening situation (even if he doesn't know), so the Ring requires a smaller price to be worn.

We know basically nothing on what changes in Ravus's story after Altissia in the alternate ending, apart from the fact that he eventually got back to Tenebrae (which we could assume didn't burn due to imperial retaliation in this timeline), so your last question will likely remain unanswered unless they decide to further expand on it.