I think you, and Schnee are getting things entirely confused. The only sustaining universe statements are Palaces, and the actual Metaverse, which was already proven to be a universe (see Yaldabaoth’s profile). Rulers sustain their Palaces, Yaldabaoth sustains the Metaverse. That’s why in Scramble, it’s gone, because Maruki & Yaldabaoth are defeated.
Umr... is though?? He’s sustaining every single universe, cognitive and physical from merging into one gigantic mesh of every single reality within Persona. That’s what he’s doing, the CU was created by humans, and Umr makes sure nothing merges together unless he is unable to do that.
Also, yes, Mementos being comparable in size to the CU was proven, as it was stated by Morgana. Again, Mementos was never merged with reality, the Metaverse was, which is why it’s never stated to be destroyed, only the Metaverse. And yeah, Mementos is the core of the Metaverse, that has been stated. However I’m not sure how this changes anything.