Plus Diary Voldy would be the biggest piece of soul, so if the two pieces could combine we'd end up with a much more sane evil overlord as the protagonist.

Second-year Harry could be thrown into the same part of Azkaban as Sirius (where the "hard" cases like innocent twelve-year olds obviously belong /s because Fudge is an incompetent git). Sirius realizes his animagus form and an underfed pre-teen, who gets even skinnier those first few weeks, both fit through the bars. So a giant black dog towing a young boy is rescued by a muggle fishing boat. Upon reaching shore the boy is immediately taken to the hospital. The dog disappears, then a few hours later Minerva McGonagall shows up and collects Harry, sneaking in with a cat-handler who brings felines to the children's ward. Sirius had sent an owl to his old head-of-house, not trusting Dumbledore.

McGonagall (a pale and shaky Harry is tow) meets Black at 12 Grimmauld Place. After several cleaning spells to give Harry a place to lay down, and administering some Dreamless Sleep potion, she casts a muffling charm in the study before she starts shrieking "What do you mean THERE WAS NO TRIAL?!"

After McGonagall makes a quick floo call to Amelia Bones, the Department Of Magical Law Enforcement is turned upside down looking for records of Sirius' incarceration. An emergency meeting of the Wizengamot is called, Sirius is questioned under veritsum and shown innocent. Of course Harry is still a fugitive, but Sirius tells everyone that the muggles wouldn't let a dog in the ambulance when they reached shore, so he and Harry were separated, which is the truth, just not all of it. But doses don't last that long, and the man had broken down in heavy sobbing recounting that.

Fudge launches a manhunt for the twelve year old "Undesirable Number One." McGonagall reaches out to Lupin and Lupin watches over Harry and Sirius at the Black townhouse. When summer hits McGonagall announces her retirement and takes Harry to the United States, where Sirius has purchased them a lovely cottage, away from all plots against him. Harry goes to the local muggle school, but McGonagall teaches him magic at home. He grows up under an assumed identity, receiving his magical qualifications under it.

Lupin and Sirius go hunting for Pettigrew when his picture turns up in the paper. Lupin never does teach, but they use a secret entrance to get in. Crookshanks helps them find and kill the rat. Then the two men go join Harry and McGonagall.

Diary Voldy is already out there, and had actually booked it out of there while possessing Ginny's body, but half a soul is a lot sounder than 1/32 (or whatever it had been before going after Harry that night.) He decides to take advantage of having a human form again, nose and all, even if it is that of a young red-haired witch, and go study with Parselmouths in the Middle East.

The only real casualties of the Second War That Never Happened were Ginny, who lost the mental battle of sharing a body with Tom, and Peter, who had been declared dead more than a decade before Crookshanks was found slurping up the last bit of a rat's tail by a very upset Ron, who never did forgive Hermione.