SPOILERS !!
Yes, upon reopening, Sans feels the player's SOUL and realizes that it's possible that the player is here to RESET. And at that point, he has accepted it, even though it hurts him. And yes! During Act One, the echo flower blurts out one of Sans' codewords. It's not the codeword that he ultimately tells you to repeat back to him. However, it's foreshadowing to him telling you one in the future once he trusts you. :)