Your welcome.
Spoilers!
Yes, Flynn becomes the monster that they saw.
The little kid at the end was Jenna, she sees the monster he most!
I just can't hate Leo! He became obbsesed over Chase because as a maned wolf he would mate for life. Also Chase didn't treat him very well and broke up with him in a pretty bad way, then the forces of Echo twisted Leo's thoughts!
About the possession and love, its complicated. Leo already had a crush on Chase before he was possesssed.
After thinking about it and go through the routes again, it seems Chase really was himself, just altered by the entities.
Leo loved both versions of Chase, and never seemed to notice the difference.
I could be wrong about that though!
Anyway, I think Flynn's ending and TJ's endings are the most tragic, and Leos ending is kinda depressing!
I wish there was a better end for Flynn and an end where Leo doesn't get mental trauma and stays friends with chase.