How one defeats wizard with the Deathstick in a duel where Deathly Hallows lore says that you cannot lose if you wield it
Yes, but you forgot the wand-lore. Harry defeated Malfoy. Malfoy was really the one that truly defeated Dumbledore, as he had disarmed him. Thus, when Harry defeated Malfoy (who defeated Dumbledore who possessed the Elder Wand), he was the wand's true master. That is why Voldemort could never unlock its full potential (even after killing Snape). And Harry's wand was not used against him.
Also, as Hermione has touted several times throughout the book, it's not just the wand but the person using the wand.
Any other huge plot holes you want filled?