2. What Hagrid Overheard
Harry is talking to Hagrid after the poisoning of Ron, and Hagrid lets it slip that he overheard something he shouldn't have:
I was comin' outta the forest the other evenin' an' I overheard 'em talking -- well, arguin'. ... I jus' heard Snape sayin' Dumbledore took too much fer granted an' maybe he -- Snape -- didn' wan' ter do it anymore ... Dumbledore told him flat out he'd agreed ter do it an' that was all there was to it." (HBP pg 405/380)
This clue comes right out and tells us that Snape is following the orders of Dumbledore, although we now see whatever Snape has promised to do for Dumbledore is difficult or unpleasant.
We also now have comfirmation that Dumbledore has a plan, which involves Snape, and Dumbleore is confident that the plan that Snape has agreed to will proceed.
3. No DADA Teacher Has Lasted More Than 1 Year
For every Hogwarts year that is documented in the Harry Potter books so far, it stands out prominently that there has been a different Defense Against The Dark Arts teacher each year.
They even joked about it at the beginning of the previous book, when the kids are looking over their just arrived book lists for the year, and they're wondering who assigned the Slinkhard book, as it meant Dumbledore had found a new DADA teacher:
Fred told Harry ... "Dumbledore was having real trouble finding anyone to do the job this year." "Not surprising, is it, when you look at what's happened to the last four?" said George. "One sacked, one dead, one's memory erased, and one locked in a trunk for nine months," said Harry, counting them off on his fingers. "Yeah, I see what you mean." (OotP pg 161/146)
And in Half-Blood Prince, Harry, Ron and Hermione talk about this again when they are surprised at the sorting feast that Snape will be DADA teacher that year:
"Well, there's one good thing," [Harry] said savagely. "Snape'll be gone by the end of the year." "What do you mean?" asked Ron. "That job's jinxed. No one's lasted more than a year...Quirell actually died doing it..." (HBP pg 167/159)
Ironically, it turns out, Harry was literally right. When Tom Riddle returned after a ten year absence to again request to be a teacher at Hogwarts, Dumbledore rejected him, and upon Harry's questioning, Dumbledore gave us some important insight into the Hogwarts DADA teacher situation:
"Was he after the Defense Against the Dark Arts job again, sir? He didn't say..." "Oh, he definitely wanted the Defense Against the Dark Arts job," said Dumbledore. "The aftermath of our little meeting proved that. You see, we have never been able to keep a Defense Against the Dark Arts teacher for longer than a year since I refused the post to Lord Voldemort." (HBP pg 446/418)
So, Dumbledore himself is admitting he knows that Voldemort cursed the DADA teacher job. This means he knew about the curse when he assigned Snape to the job this year. But he never intended Snape to be in the job for longer than a year to begin with, as his plan for Snape to kill him and flee at the end of the year must have already been in place.
5. Severus... Please...
If you believe that Snape is acting on Dumbledore's orders to kill him (or possibly just make it look like he killed him, although he'd probably still be hurting him), then Snape's demeanor and Dumbledore's final words take on a whole new meaning.
...somebody else had spoken Snape's name, quite softly. "Severus..." The sound frightened Harry beyond anything he had experienced all evening. For the first time, Dumbledore was pleading. Snape said nothing, but walked forward and pushed Malfoy roughly out of the way. ... Snape gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the harsh lines of his face. "Severus... Please..." (HBP pg 595/556)
In that passage the reader is supposed to believe that Snape hates Dumbledore and feels revulsion for him.
But to help us understand the real meaning of Snape's feelings of revulsion and hatred, J.K. used almost the exact same words for what Harry was feeling just one chapter previous:
"You...you can't stop, Professor," said Harry. "You've got to keep drinking, remember? You told me you had to keep drinking. Here..." Hating himself, repulsed by what he was doing, Harry forced the goblet back toward Dumbledore's mouth ... (HBP pg 571/534)
Even though Snape was to kill Dumbledore on Dumbledore's orders, it must have been something that was still really emotional and difficult for Snape to do, exactly as it was for Harry to make Dumbledore drink the potion.
The feeling of revulsion on Snape's face was not for Dumbledore, but the act he knew he had to commit. The hatred was not for Dumbledore, but for what Dumbledore was making him do.
And when Dumbledore said, "Severus... Please..." he wasn't begging "please don't". What he was really saying was, "Severus, please kill me, as you promised you would."
Way back at the end the first book, when Dumbledore confirms for Harry that Flamel would die now that the philosopher's stone was gone, Dumbledore explained:
"To one as young as you, I'm sure it seems incredible, but to Nicolas and Perenelle, it really is like going to bed after a very, very long day. After all, to the well-organised mind, death is but the next great adventure." (SS/PS pg 297/215)
Does that sound to you like someone who would beg to save his own life?
And these are just fascinating... I agree with evey single word said, every single hint. Excellent ✅