Great ideas here
I don't usually comment on stories at all. However, as a college composition professor, I have some constructive criticism for you. You're so good already that it's worth the chance my concrit won't be received well.
You have great ideas, a very vivid imagination, and a nice grasp of characters. I particularly like your idea about Fawkes' sacrifice and your use of Aberforth.
You show skill at expressing your ideas, but there are a couple of issues in your writing that are holding you back from being your best.
1. I think you would benefit from changing your verb choice. If you cut back on your use of being verbs (is, was, were, and other variants of "to be"); it will eliminate wordiness and make your prose flow more smoothly, as well as making it more exciting.
For example, compare:
"Harry was sitting at the table. He had finished his breakfast long ago, but he hadn't left the room. Thoughts of dreams and the letter were occupying his mind, now that Molly had left him to it"
to a possible revision:
"Harry sat at the table without leaving the room for a long time after finishing his breakfast. His worries occupied his mind, now that Molly had left him to it."
You get the same ideas using most of the same words, but it's tighter and you get more punch per word. The change also improves rhythm; if you make it, you'll find your sentence structures growing more varied and less choppy. There are many effective ways you can reduce your use of being verbs. Given how good a writer you are, I know you can find a great one of your own.
(By the way, the helping verb "had" can become awkward too if you use it frequently when you're putting being verbs into the past tense; try not to overuse that construction. This should resolve itself if you reduce the number of being verbs you use.)
2. Polish your sentence punctuation. Most importantly, you need to eliminate run-on sentences. Be careful in looking for places where you need to use a sentence-ending mark, such as a period or question mark, instead of a comma:
Instead of:
"What could Ginny reply to that, she loved her family, and she knew how much family meant to Harry."
try:
"What could Ginny reply to that? She loved her family, and she knew how much family meant to Harry."
You get your semicolons right most of the time, but don't let run-on sentences slip past you.
3. Work on showing (using concrete images), not just telling (using abstract or general words), your story. Here's a passage in which you tell several of things you could show, and as a result, the content loses a lot of its potential to impact the reader:
"No, he had been forced to entertain himself with far less desirable people with repulsive personalities to match their bodies until he'd decided to stop doing it entirely. But those thoughts were very far away at the moment as he felt Charlie fingering his entrance."
What you lose in length if you reduce your use of being verbs, you can pick up by adding more show to your tell. A good beta won't presume to rewrite your story for you, but just for the sake of example, I will give a brief possible interpretation of that passage, with some "show" rather than all "tell:"
"Severus shuddered, remembering the repulsive personalities and bodies he'd experienced: Lucius's love for making his partners scream and beg for death, the loathsome scent of Voldemort's corpse-like skin as Severus knelt to receive his favors: sickly-sweet with rot, like the breath out of a grave.... Charlie's finger slid across his entrance, jolting him back to the land of the living."
Good as you are, I'd love to see you get even better. A tough grammar beta can help you with these things.
I'm genuinely sorry if you're upset by this comment. It's my intention to help you, not attack you, though I'm aware that some people prefer not to get concrit and may regard it as an attack, or may simply find it irrelevant. If you dislike concrit and you're having fun, by all means continue to enjoy writing as you write. That's the most important thing. :-)