I really love how you phrased this especially the point about Jeremy not being healed and unable to be completely free from Greg.
I too hope he'll thrive one day and free himself of these shackles but like in real life, some victims never truly heal and keep these shadows for the rest of their life.
That's precisely why this manga hurts so much, because it depicts the cruel and brutal reality of csa victims.
It doesn't attempt a "happy ending" for the readers sake but an open ending to interpret as hopeful if the reader chooses to.
Reading your explanation also reminded me once again why I never hated Ian, he's but a child as well and thrown into a whirlwind of confusion and guilt, facing off against Greg's shadow and the responsibility he feels towards Jeremy and his suffering because of it.
I think this is the best realistic depiction of csa and it's aftermath I've ever met with in manga, I was NOT prepared. I gotta say I wanted to strangle Ian with my own hands at many points in the story, but on the other hand I understand. It's not like he's equipped to deal with Jeremy's pain, he's not a neutral observer disconnected from the situation, a professional with a therapy plan. He's just a person fighting Gregs image looming over him similarly as Jeremy does, an imperfect person with his own struggles. I very much like the ending in particular, it's not like Jeremy is healed, he's not free of Greg and might never be. It's painful and real, but cathartic in a way, he survived, and he continues surviving and one day he might be able to thrive.