Most ppl focus heavily on Law's Immortality Surgery as the only known Ultimate Ability of devil fruits, but we see another one mentioned in Dressrosa. It seems like it's more common than we might think that the user of a devil fruit can sacrifice their own life to perform a hidden ultimate technique. Law can give someone immortality. Princess Mansherry can apparently permanently restore damage to entire buildings (or more), not just temporarily heal people. What if we've already seen a third?
I think the Soru Soru no mi's ultimate ability is that you sacrifice your own life to take bodily possession of another human being, and Mother Carmel assumed control over Mom's body. Big Mom = Mother Carmel but Big. O-lin in Wano is like, literally the same personality as the little kid Linlin from Elbaf, with no development or change. She's a child's brain in Big Mom's body, who never aged a day since Carmel took her body over, and all she can think about is food and helping people, the same personality she had back in the orphanage.
Remember the first time O-lin shows up is after Big Mom is completely submerged in seawater, deactivating her/Carmel's Devil Fruit powers. She remains Linlin until Queen knocks her body unconscious and Big Mom can retake control.
I think what might have happened that day is that Linlin really did just eat the semla.
Carmel got the party set up, sold most of the kids to CP-0, then either they wanted the kids for free or they refused to buy Linlin. A conflict arose, and CP-0 being who they are, they started busting out rokushiki moves and killing witnesses. At that point Carmel said fuck it, if i'm going to die anyway get ready, then possessed Mom and started trying to murder the agents/drive them off. One of them managed to give her a wallop on the head or hit her with some sea prism stone weapons, Linlin woke back up eating semla and didn't remember anything. Streusen comes over, starts being a groomer, and then sometime soon after that, Carmel regained control and started building her criminal empire and reverse harem.
When we see this, shit gets real.