I've looked through the memories and found partial answers to my questions: Legal adoption and Children's homes; however, my questions are a bit more specific, so please bear with me.
It's clear from canon that the Dursleys subjected Harry to verbal abuse and neglect. IMHO, Vernon and Petunia didn't hurt Harry physically, only Dudley did.
What I don't know is, in the 1980s, before Harry was at Hogwarts:
* If a well-meaning adult noticed the Dursleys treatment of Harry, what could they do?
* Would there need to be signs of physical abuse before anything legal could be done about it?
* If the Dursleys were accused of child abuse, what would be the procedure? What would happen to the Dursleys, to Dudley, and to Harry?
* If the Dursleys were convicted of child abuse, what would happen to Harry? (And to Dudley?)
* If Harry was put up for adoption, could specific people petition to adopt Harry? Would it make a difference in Muggle law if they were related to Harry?
Specific examples: Aunt Marge is only related by marriage, because she's Vernon's sister, and Vernon married Petunia, who is Lily's sister. Whereas, looking at the Black family tapestry, if Charlus Potter was Harry's grandfather, then Narcissa Malfoy (nee Black) and Andromeda Tonks (nee Black) are Harry's second-cousins.
* If there were provisions in Lily and James's wills, would that make a difference? What if those provisions had been ignored (when Dumbledore put Harry on the Dursley's doorstep)?
Sorry, that's a lot of questions. But each one followed on from the other...