What if the Mohg we fight isn't Mohg because Miquella snatched his fucking body? No really, I was always kind of operating like Mogh was, in some capacity, brainwashed by Miquella- the softness of the voice, the obsession, the fact that Miquella is associated with brainwashing items so potent that, under the right circumstance, they can even cure Frenzied Flame lordship. But then a thought occured while seeing Mogh's intro again: do we know what Miquella is actually capable of? We've never seen him in action, the most we can gather from the world is the Haligtree and the possibility of becoming an Empyrian, but nothing of Miquella having any kind of limitation in that ability beyond what a mere crafted replica of his power can do.
"So what? Mind control doesn't mean he can swap bodies with people because if that were true he'd swap to an adult's body." This is half true in that if it were necessary, he COULD swap bodies if he had the ability. If he did so, however, he'd leave his Empyrian-ness behind and have no claim to the throne. After all, who'd believe some Haligtree soldier is the child of Marika and Radagon? Not only that but shard bearers and attempted usurpers of his claim would be powerful people such as Ranni and the shard bearers he doesn't know your character has killed, so he needs a powerful body to take with enough influence to make the claim seem legitimate, even if it's through a declaration of being a consort and a tenuous connection to other Elden Lord candidates.
Enter Mogh. Assuming Miquella is indeed Saint Trina and his influence can go far as to affect the dreams of distant peoples, he could've easily had Mogh think it was his idea to steal Miquella in the first place by planting the thought in his dreams. While similar to Miquella simply using his influence on Mogh, not only does Saint Trina have much more reach when it comes to direct influence, needing only slumber to captivate people, using Mogh's dreams to influence him makes it so the plan isn't known by Malenia, who wants to protect her brother. She believes the Haligtree can fix them while Miquella has been reported to be the doubter of fundamentalism, so it stands to reason that if one of them would doubt the Haligtree it'd be the one who had doubts of the Golden Order, spurred on by the lack of help it gave his sister.
So he gets Mogh to 'kidnap' him in order to leave the Haligtree and get a suitably powerful consort. If Miquella were able to take over minds it'd happen now; the body in the egg/cocoon while still an Empyrian is more or less deformed judging by the arm. What makes me think Miquella has taken over Mogh's body though- he doesn't say so nor does Mogh use mind bending powers.
The reason why I thought of it was, again, the intro to Mogh's boss fight. He pours out of Miquella's blood, speaks softly of having to fight, then growls at the Tarnished about the Mohgwyn dynasty. The voice in particular struck a chord the first time I've heard it- the soft voice sounds much more natural, as if Mogh were accustomed to being quiet. Meanwhile the voice he addresses the Tarnished with is oddly childish: rather than a booming voice being yelled from the chest, he growls from the neck as if he's a child pretending to be a monster. The growling is almost a caricature of evil voice acting done by someone who really never felt real malice, like maybe someone sheltered and protected by a stronger person due to circumstance...
It's not just that though, another bit of evidence is that Mohg NEVER, at any point in is fight, uses his omen powers like Morgott. Morgott uses everything he can, from Omen fire enwreathing his sword, to Omen body fluids reacting to his holy magic, to even the accursed bloodflame his twin brother has. And yet nowhere in Mohg's bossfight does he use Omen's fire despite it likely being easier to cast due to him being biologically predisposed to using it, instead using the bloodflames- almost as if he didn't know about the powers Omenkind possesses. Mohg was also in Miquella's bloodstream. Yes it was so he could influence Miquella's blood and body to become accursed for the Formless Mother, but if a demigod can influence a god's body from within surely the opposite is true. He is leeching off of Miquella after all, and maybe his mind was slowly taken from him due to the exposure to Miquella's magic.
TL;DR It's possible Miquella used his half brother and his Outer God to become Elden Lord without having to be in a child's body and possibly looked to help his sister by looking towards the Formless Mother. Mogh's voice is very off-putting for what Mohg is, he never uses all of the powers he could use in his boss fight, and being in a hypnotizing god's blood may have more consequences for you than for them. It was also set up to make Mogh's attempt at Elden Lordship very public and having a show of power- that he can defy the rot goddess that ended The Shattering in a stalemate- without putting any suspision on Miquella via influencing his dreams and convincing Mogh it was all his idea.