Adult Helen isn’t much of a character—her role mostly boils down to confusion and denial, so her main job in the episode is to—sometimes accidentally, sometimes with unconscious intent—not know what’s going on. It’s a fundamentally passive part, with the lion’s share of her character’s agency manifested in Markie.

Maybe I'm just trying to see the best in an episode I also don't like as much in execution as in ambition but this kind of works for me? It's like by repressing her memory of her trauma instead of working through it, Helen has given up whatever power she had over her life and situation. Like she's left the reality of what happened in Selden's hands to mold as he will and so given him the upper hand in their present day interactions. Markie is the one with agency because she's the part of Helen that hadn't yet given it up. She's manifested not only to make Helen remember but, in doing so, to give that agency and power back to her.