Hey, @bryan-imhoff.
Yeah, it could be Mephisto, I guess. I read somewhere else that they thought whoever he is was out of character for that particular Pietro and I didn't think it was that big of departure myself. Still very funny, and not at all like MCU Pietro, or comic book Pietro for that matter. Of anyone, though, since he just showed up out of nowhere, it makes more sense that it be Mephisto or someone else than any other character to date.
I'm just not sure about the line of questioning. Why ask her how she did all of the reality building if he's trying to keep her under? The more he asked, the more it made her think about her situation. So, I don't know. If we're incorporating Fox characters into the MCU, though, Pietro's a good place to start, however they end up explaining it. :)