We know from the books that Severus Snape always wanted DADA teaching position.
Is there any canon (books/interviews/pottermore) confirmation of why exactly Dumbledore didn't want that to happen in the first 5 years (or, for that matter, even before Harry went to Hogwarts, as the DADA teachers changed every year evers since before the First Wizarding war, when Voldemort came to seek the job)?
I can think of 4 possible reasons, but am interested in actual canon confirmation:
To help maintain illusion that Snape wasn't fully trusted, for Voldemort's sake
To help maintain illusion that Snape wasn't fully trusted to those who didn't trust an ex-DeathEater Snape, and would have made a lot of noise and complaints had that happened (I'm sure Barty Crouch Sr. would).
To avoid any risks to Snape as a chief Voldemort mole (as in, the 1-year-curse of the job could have caused Snape some damage, like it killed off Quirrell or permanently damaged Lockhart).
Because Dumbledore - just like he didn't trust himself with power of being a Minister of Magic - didn't trust Severus Snape who used to be "up to his ears in Dark Arts" to be too close to Dark Arts again. As in, he was worried that Snape may be seduced by the lure of the Dark Side.
when Professor Dumbledore took Professor Snape on to the staff and Professor Snape said I'd like to teach defence against the darks arts please and Professor Dumbledore felt that that might bring out the worst in Professor Snape
(transcribed from a recording)
So, your reason #4 seems to be correct. From canon it's unclear if any of the other reasons you've described come into play.
I'd say the combination of #4 (didn't wanted Snape too close to Dark Arts) and #5 : he knew the job was cursed (he says it himself in Half Blood Prince :
"Oh, he definitely wanted the Defence Against the Dark Arts job. The Aftermath of our little meeting proved that. You see, we have not been able to keep a Defence Against the Dark Arts professor for more than a year since I refused the post to Lord Voldemort."
and he wanted to keep Snape safe and at Hogwarts for more than a year (no proof on this, but it seems quite reasonnable).
Even though J.K. Rowling said that it might bring out the worst of Snape, I don't think it was reasonable. Snape had seen the worst side of him supporting Dark Magic and by extention, the Dark Lord, when the only love of his life died, trying to protect her child. So he wouldn't have gone back to his past life again so easily. I think, reason #3 would have been more appropriate and reasonable. Albus might not have wanted to lose Snape who was a valuable ally. Also, he needed Snape to keep a close eye on Potter while he was being prepared by both of them for the Battle-which-mattered-the-most. You see, Snape was always there to protect Harry except in the Chamber of Secrets and the graveyard where Voldy got a new humanly body (sans a proper nose).