Letting child rapists go unpunished is incredible amoral (not to mention illegal) and frankly insane. Since you made the Dursley abuse so much worse than canon, the repercussions much be much worse than canon - they either die or get life terms in prison. Otherwise your protagonist is really responsible for all the harm these evil people will do in the future. Basically, your Harry just pulled a Dumbledore.
Author's response
I disagree. In canon, Dumbledore made no attempt whatsoever, to punish the Dursleys. It's especially disturbing, seeing as how it could be inferred from canon that he was responsible for the way they treated Harry in the first place.
The funny thing is, in canon, Dudley is the only one of the Dursleys that even made an effort to turn himself around. It wasn't much, but it was there.
As for Vernon and Petunia, they won't be going unpunished.
Putting them in any prison in the world, would have them dead in a few weeks at best. Prisoners are more capable at information gathering, than most people give them credit for, and prisoners also have children. They really, really don't like child abusers/molesters & rapists. While that would be fast and brutal, it would leave the 'new and improved' Harry unsatisfied. He doesn't want them dead, he wants to see them suffer!
What I'm doing is effectively jailing them for life, in an environment they cannot escape, while all the while they are faced with the very people they sucked up to in the hopes of being seen as normal.
When the courts get through with them, they will have to pay back their debts, and at the same time suffer the 'horrendous indignity' of living in a house Harry owns, paying rent to him, to the tune of nearly every penny they have, and no way out.
They will be poor!
Their only other option is prison, and they both that isn't a god place to be.
With the neighbor's penchant for gossip, it will be no time at all before every single person in Little Whinging knows exactly what they have done. Effectively, they will be 'shunned'. Their social image, which is the most important thing in their lives, will hit the sewer. Services will be late, or spotty, Prices will go up for them, if they have a car left, parking spots will vanish, station attendants will slop gas on their car, If not, bus drivers won't stop to take Vernon to work...
They also will have several thousand hours of community service to perform, facing the same people they derided every day of their lives.
These are the kind of things that will drive Vernon insane. Perhaps, if he's lucky, he'll have a stroke.
Alorkin