Completely sap the environment and become... really big? Go elemental, shapeshift, and still possess a vulnerable mind?
I don't recall Meggan ever manipulating or absorbing psionic energy, and she's never dealt with telepathy particularly well in the past unless I'm forgetting something.
Besides, she's against completely sapping the planet and killing it so I doubt that her highest showings would really be applicable here.
She once morphed into a Surfer like being and wielded the Power Cosmic. IIRC she was in a foreign dimension, but it isn't unreasonable to assume she could do it anywhere.
Besides, her own empathy was strong enough to manipulate a Phoenix manifesting Rachel on more than one occasion. I think that's more impressive than being edged out by Exodus.
As for matter manipulation and tk, Meggan is basically immune. Even reality manipulator Jamie Braddock couldn't do anything permanent to her while she was consciously aware of him.
Yes, that is unreasonable to assume, as the foreign dimension was extremely magical and implied to have amped Meggan.
Manipulating Rachel via empathy has little to do if Sersi starts a telepathic battle. Do you believe that Meggan could trump Phoenix manifesting Rachel's telepathy outright with her empathy, in a head on telepathic battle?
I also think Exodus would take this fight, so I see Sersi's battle with Exodus as having little bearing on your point.
As far as matter manipulation goes, I agree. There's nothing to really support that Meggan would be 'immune' to TK, except that she could just turn to air or something I suppose and assume a body that Sersi would have difficulty affecting with TK.