Defense Against the Dark Arts situation

From www.dumbledoreisnotdead.com Every Hogwarts year that is in the Harry Potter books so far it stands out planely that there has been a different Defense Against The Dark Arts teacher each year.
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 clues 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.