Yep, definitely leaning towards Marcus as we know him being
dead dead here, with the "Sol" signal essentially now piloting his body. Three different layers to this being now, what with the original Caleb's body, Marcus' face, and now the entity "Sol" driving it all.
As for the comment a few posts above about the hanged man, I don't think any of the tarot cards we've seen in either season have been explicitly evocative of that (though the Mithraic painting in season one's Caleb and Mary flashback, of course, was), but this one strikes me as interesting yet again:
Last season we saw hooded figures gathered like this in a sort of ritual watching what seemed to be a (failed) ancient attempt at birthing a serpent, and in today's finale we saw a very similar gathering of hooded figures watching the tree transformation ritual. Both the serpents and the trees are directly linked to "Sol," so I'm guessing these flashbacks/tarot scans are depicting ancient "Believers" who underwent these trials very willingly (rather than having to be conned into their roles, like Mother and Sue were) and were opposite Grandmother's opposing Technocrat faction that sought to de-evolve humans in order to save them from "Sol."