In the books he's planning on marrying and fathering children, which suggest he has a functional Arnold Schwarzenigger. Do you think they'll keep this in the show? Will the dragoncock raise, bigger and stronger than ever, in righteousness and glory?
So by book 5 Dumbledore doesn't know about the Horcruxes but he already suspects that Voldemort is immortal and there's no point in killing him since he'll come back, I get it. But what he does know is that if his body gets destroyed, it takes him years and years (14 to be exact) to come back with full strength, so why didn't he do that? The fandom generally agrees that he is more powerful than Voldemort, and he has the elder wand, should've been hard but doable to make him explode into pieces again. That way they would've had more time to 1) figure out why he's immortal and, 2) find and destroy the Horcruxes once they figure it out. Not only would Harry and him have more time, they would also weaken Voldemort's influence, since many of his followers would lose respect for him after seeing him be defeated a second time. It would also be much harder for him to get Harry's blood for his resurrection spell this time, now that they're warned.
I get it that during that duel Dumbledore also wanted to stall for time until Cornelius arrived and could see Voldy with his own eyes, but come on! You have a bunch of death eaters tied in the department of mysteries, just use veritaserum on them or extract their memories!