I remember something similiar to the 2nd story you are looking for, but it had sev who had been deaged from a potion accident, i think, but he was sent with harry to sirius and sirius abused sev.
And as harry didnt know what sirius was doing was abuse, he thought it was normal cause of how he was treated by vernon, when harry and sev went back to hogwarts, dumbledork believed sirius and wasnt the nicest to sev, sev went to his quarters and tried to kill himself.