Miquella is framed as being both a brilliant strategist and emotionally childlike. That bloodflame attack looks like someone's about to have a tantrum.
We see in the haligtree when miquella didn't get his way he just abandoned it, didn't even tell anyone he didn't bring with him, and them I assume are stuck rotting in coccoons in the haligtree. He probably didn't even warn them first, one day you're a misbegotten crusader proudly defending the haligtree the next you're stuck phantoming into the stinkiest hole in the world to make sure nobody finds Trina.
I don't think miquella can mind control other demigods, or if he can it requires multiple "grab attacks". I think radahn said no and mogh said yes. I don't think mogh knew what he was saying yes to at the time, though. "Grab you from the tree while malenia is negotiating with radahn to re-establish the Old Order with my glorious hornsent corpse? Bet."
I don't think miquella knew what he was saying yes to at the time either though, wrt mogh.
If the gloam eyed queen died before miquella was born he may not even realize that she became the formless mother. We gotta remember that these are aspects of the story that Gideon hasn't even worked out.
Or maybe he does realize, and thinks that injecting the formless mother (the twisted, vengeful god that the GEQ seems to have turned into) into the order is the same as restoring the GEQ.
So there you go, what was being framed as incest is actually cannibalism.
But still and all, why didn't miquella pick like any other demigod for the vessel?
Morgott, obviously, would see this as a Worst Possible Idea, as morgott seems to think there's something particularly dangerous about omen/hornsent blood.
Especially when it's lit on fire. He even changed his own cutscene over it.
