And from what I know of the character, MM should have the speed and strength edge.. Even if Sersi used the old adamantium box trick, he could probably phase shift out of it..
However, we know MM's brain gets affected from shapeshifts, so if she managed to transmute him into something that wasn't intelligent enough to transmute back...
Depends on the method of reaching intangibility. For instance, Sersi should have no problems ripping apart someone like Shadowcat, a character who pushes through matter while still retaining physical form.
As for the other dimension thing, Sersi has ripped open dimensional portholes while her own energy was feeding back on her. Not sure if she could reach Jonn without having to do that and I'll concede that it strains her. Conversely, she can phase too.
Her psionic prowess is expansive. She effortlessly scanned the minds of everyone on earth, stalemated BT Exodus right after opening a dimensional porthole, and has mind controlled the likes of Thor and fellow Avengers.
Sersi has been KOd by blunt trauma in the past, so it's possible.
She has intangibility of her own though and her shields have halted universe shattering blasts. I'm not being hyperbolic, either. The entire universe was destroyed and the only thing to survive was the area she was shielding.
She didn't shield a universe destroying blast. She shielded the area where she was and kept the energy in that area from achieving it's destructive goal. In order to have shield universe destroying energies, she would have had to been at the epicenter of said blast and stopped the destruction or shielded it from releasing.