Skyrim!Sheogorath cannot be the same old Uncle Sheo we all know and love
I believe I've stated a few times that I'm not a big fan of mantling on Shivering Isles theory, but I feel like I need elaborate and write my thoughts down on the theme
The narrative of reoccuring mantling was brought up by ESO, known for it's lore butchering and overall timeline fuckery. And in this case it's the same old "we know there's a Big Theme so we'll roll with it without unique and complex understanding of it" case
For me it's a cheap writing. The whole plot of Shivering Isles is that Sheogorath creates a new way to defeat his sworn enemy - so fresh, in fact, that he doesn't have time to think it through, Haskill has to follow it and deal with it instead.
Sheogorath's new brilliant idea is to give away power [to a mere mortal!] - a pretty big move for a daedric prince, don't you think? Daedric creatures are prideful and will never do such thing. But he's not bound by any universal, daedric rules, even if it's the rule of pride. He has a free will, free even from the limitations of mind and temper.
That what defeats Jyggalag. Sheo's been right all along - free will is stronger than any predetermined fate.
A beautiful leitmotif, the universal wisdom that it's only you to decide your fate, you can do whatever you want and create yourself a version of life you desire...
Yeah, all of that is thrown straight to the trash by ESO writers.
Turns out, Sheogorath is just as stupid as Jyggalag is, trying the same method again and again in hopes that maybe one day it will work (It's a flag that Jyggalag may be insane himself and very important to his character, please for the love of god don't rob him of it like that)
One day it works because of ??? reasons and that's it. New amazing Shivering Isles interpretation.
Yes, Sheogorath can replicate Jyggalag's methods and try the same old thing forever (he's Mad God, after all), but why is our Champion so different from previous "mantlers"? Ocs' stories can weave into this narrative but there must be divine intervention to pick them as The Chosen One to solve little mad prince's affairs etc etc. Why would they do that... I can't believe I'm actually writing this.
This writing decision does both of daedric princes dirty imo. It's a story we've seen in some other media and it's easier to understand, but it sands down the tragedy of this plotline and important lore occurence.
Sheo knows that the story repeats itself and it will happen again and again, and the pain of his loss is real. It's not about being doomed, or weaker than opponent, it's about things he's used to and grew fond of, being taken away from him in a tactical defeat.
Throughout the whole SI plotline Sheogorath sends us on a different missions - mostly to fix some stuff, build barriers and unite his soldiers. It's not pointless tasks to game with the hero - it's what Sheogorath usually does to defend his realm. That's why it doesn't work. You cant defeat someone with outer forces when they're literally in your head.
So no, nobody mantled Sheogorath before his Champion/HoK. Not in the original Oblivion narrative.
Haskill did not Mantle™ Sheo, but I do believe he did something of sorts. Not the thing the hero does in the SI, but there's enough freedom to make something up.
People often forget that Jyggalag is not just part of the Madgod, he is him. Sheogorath is him, transformed and corrupted. Jyggalag is the host, as some may say.
After events of SI Jyggalag is gone. His fate is of no matter, what we have left is the soul of madness, the essence of it. The realm is a part of the prince and it was left relatively unharmed, so while all of their skirmish happened something new and standalone had managed to form.
It has never been independent without a person-host (Uncle Sheo), so it'll take time to take over the hero. They will become the Madgod - the title, the role, but not the person.
Having Jyggalag as a part of himself was essential to Oblivion!Sheogorath as a person. You can even interpret his Three Heads symbol as Jyggalag - Madgod - Sheogorath (mania - something in-between - dementia, mind - soul - heart etc.)
Something something a fair share of mental problems come from fear of death and Sheogorath can basically understand how it feels like, he can feel it through himself, while other daedric princes are not affected by it.
That's why I tend to see Skyrim!Sheogorath more as a force of madness (force of nature almost) with human memories, rather than a person.