Voldemort did use the killing curse on him in GoF, but unfortunately for him a wand is not able to kill the master of its brother, i.e. Harry couldn’t have been killed by Voldemort’s initial wand after Harry received his Fawkes wand in book 1. However, once Voldemort took Harry’s blood, it was never again possible for him to beat Harry’s Fawkes wand regardless of what wand Voldemort used. In the graveyard fight, Harry won because of his courage from having “had accepted, even embraced, the possibility of death, something Lord Voldemort has never been able to do”, which caused Harry’s wand to contain “the power of [Harry’s] enormous courage and of Voldemort’s own deadly skill: what chance did the poor stick of Lucious Malfoy’s stand?” re: King’s Cross

Dumbedore didn’t know that Voldemort planned to make 7 horcruxes but he still knew that Harry was a Horcrux, which he used to refer to as Voldemort having ‘transferred some of his powers to Harry’ prior to the 6th book