A change that I've made that I should probably address here is the dementor lifespan. We really don't know much about dementors from the books, so I took license to make them more like creatures than like an immortal demon.
They are natural creatures of the world that serve a purpose in my story, extremely misunderstood, but natural all the same. As a natural creature, they do eventually die. They cannot get sick, grow old, or be killed, but their magic eventually fades, and they cease to live.
Theoretically, they and all the souls they had consumed would then pass into an afterlife. They will always live to reach the top of a wizard's lifespan since they cannot die from anything besides natural means, but they will eventually die just as all creatures in the world.
If you notice, my vampires are not immortal either. This is to distinguish the creatures of the world from what Voldemort is unnaturally doing to himself in twisting and breaking his soul.
Also, I don't see immortality as a blessing but as a curse to see all you know or will ever know grow old and die around you. If the dementors were trying to help Harry, I wouldn't want them to curse him in the process.
Lastly, this all has the added advantage of making it to where, if Harry takes care of Jamie, he will for at least most of his life be able to walk and interact in both worlds.
This is my reasoning, whether you agree with it or not, I hope you can enjoy the story!