I literally got to this manga three days after the last chapter dropped, and I read it in one sitting. Yeah... it's a mess. Pity. It started with much promise. From my vantage point, around the time they got to the abandoned research lab behind the shrine is when the story went off the rails. From then onward, the remaining half of the story basically fizzled out.
Assuming that Marika and Other-Marika could never meet face to face, it would have been cool to see them leaving messages and/or clues to each other; we could have seen pleading, negotiation, taunting, and what have you -- all with Other-Marika as The Unseen.