One thing to note, text-wise, Mouse enters the story immediately after Harry is exposed to Lasciel's shadow. Something tells me this isn't a coinicidence. If I were Mr. Sunshine, I might makes sure that a certain wizard that seems to be tied to many important events had a positive influence in his life that balanced out the negative.
Lash was a corrupting influence, whereas Mouse gave harry positive memories to draw on, even a motivation. There is a passage in Dead Beat, where Harry is feeling like he is in over his head, and he recalls that Mouse "thought I was pretty darned cool" and it picks him up.
I don't think this was a 7 words type balancing of the scales. But more of a life-line if you will.
nice going so far Serack. there was a piece of a quote from the book you cut off i think is significant though. when you talk about his sentinel abilities you left off that he can harm spiritual creatures and his bark effects across the NN boundry. that is powerful in itself.
as to when he tells Lea he won Harry. i have always said it was meaning he won the PRR (Puppy Royal Rumble) to stay with Harry.
and the part of Blood Rights where Harry looks back at the Pups and finds that the Notch-eared Puppy had fought his way to the top of the dog pile supports your theory wonderfully.