One of the datalogs in the main game say that most of the Divine Messengers were killed in a stand-off with Naglfar during the early days of Lucis (presumably after the Astral War). In Episode Ardyn, Verstael's notes say that most of the Divine Messengers were killed in the Astral War because they sided with Ifrit (this is the JP version; the EN version changed this to say that only Ifrit's own messengers sided with him and died). Some Japanese players have speculated on 2ch that this is actually a hint to something bigger in Dawn of the Future, because it doesn't make sense for the other messengers to rebel against their masters and side with Ifrit. By the time the events of XV take place, there are only like 3 or 4 messengers left, if the other 20 were slain for siding with Ifrit that makes the Astrals even more suspicious imo.
So I'm wondering if this alludes to Ifrit basically being screwed over by Bahamut in a similar way to how Somnus screwed over Ardyn. Did Bahamut manipulate the people of Solheim so they'd turn on Ifrit? Was Bahamut jealous of Ifrit being worshipped by Solheim, and so he wanted to see it destroyed so he could replace it with his own Kingdom, Lucis? Bahamut is the God of War in XV, I could see him being the one who actually started the Astral War and not Ifrit.
A slight correction as to the number of Messengers remaining at the time of FFXV: we know of at least five living messengers (Umbra, Pryna, Gentiana, Carbuncle, Avatara) and one "dead"-but-revived messenger (Garuda). Bismark is never said to be a Messenger directly, but he is given sufficient divine mystique to make it a possibility. Episode Ardyn suggests yet another potential living Messenger -- the "Dragon of Taelpar" who Verstael connects with a legend in which Bahamut guarded Somnus with "his dragon at his side." All in all, we could know of up to eight Messengers, seven of whom were alive at the time the game began.
...come to think of it, perhaps the "Dragon of Taelpar" is Enkidu?
Anyway, the idea of Bahamut pulling a Somnus and intentionally manipulating things to make Ifrit go rogue is absolutely out of line with the themes of the game proper, and I would absolutely write such a development off as pure AU if they attempted to foist it on us. >_<
All the cutscenes with Aera and Somnus I assume exist just to show you where Ardyn's mind is.
Now, he did accidentally turn her into a daemon in that one scene right. And that's why she had to be killed before becoming one. Did he do this by accident or was it subconsciously?
Actually, I'm not sure we're meant to see hallucination!Aera as having been accidentally turned into a daemon. Her dialogue makes it sound more like she was cursed by the gods for defying them than that she had somehow become infected.
I'm inclined to think of hallucination!Aera and hallucination!Somnus as two parts of Ardyn himself rather than as true representations of Aera and Somnus. Hallucination!Aera reflects Ardyn the healer, as presented by the Scourge -- she represents Ardyn's suffering, his feeling of being cursed by the gods, his continued willingness for self-sacrifice, and his desire for death, but doing as she asks will leave Ardyn bereft of all of his innate goodness (hence his unwillingness to obey). Hallucination!Somnus, in contrast, reflects Ardyn as Adagium -- there's a reason why his demeanor is almost identical to Ardyn in his cruelest moments in the game. By killing hallucination!Aera, hallucination!Somnus forces the transformation from Prologue Ardyn to game Ardyn.