Charlemagne inherited a large portion of western Europe and then went conquering for the rest.
Magnus the Pious was minor nobility but managed to convince multiple warring Emperor-claimants to unite the country under his rule.
He was also on a divine mission. That's a lot of heft even before literal intervention. A peasant girl convinced she was on a divine mission and able to convince others of it is the reason France exists as a country separate from England. Getting them moving isn't actually that outstanding, and after winning a big war against the literal forces of evil it wouldn't be a hard to keep it. A divine mandate and the loyalty of an army is about twice what you need for that.
The aftermath of his death would be the make or break point, but there's a few factors that would make it a lot less fraught than you might think.
-First of all, he reigned for 65 years (Lexicanum tells me). That's huge. Just about everyone has never known anything different, and the divided empire would be a story from their grandparents or great-grandparents stories. And mostly in the context of "and then the Great Ruler stopped that and united us".
-Second, it was still a (mostly) united culture with the same gods and the referring back to the same history.
-Third, there would be some forces very invested in keeping the Empire together. First and foremost, the Wizards, who's ability to live unburned is directly tied to the existence of the empire. I could say something that suppressing separatist ideas in the aftermath of Magnus' death is where the Greys really turned themselves into the boogyman of the nobles, and it probably happened a few times and it could've gone that way, but I honestly think the fact that Magnus reigned as long as he did buried most problems before the Greys had to.
Also, they didn't do a very good job stopping the separatist ambitions of Marienburg, so who knows how good they'd have been at that.
So yeah, I think it's actually pretty plausible, especially because Magnus ruled 65 years holy shit. I hadn't realised that before I checked, but that goes a very, very long way. Like, that puts him comfortably in the top ten of IRL longest reigns.