I took that as her being corrupted by the mirror world and Not Scott losing the corruption of the mirror world, but it could have been done more tidily.
I would rather have had the letter threaten to do horrible things to her, then have Christine make shit up as she gets more corrupted and desperate, and then while Not Scott is chilling with the pug doing some soul-searching Scott comes in and kidnaps him like what happened before, throws him in the mirror world, and they never see each other again. Maybe even have the question of abuse be held off until the end when the dust settles and have it be revealed, fucking with Scott's feelings about everything.