Sure, I understand that. But think of it this way: if you follow the method that @BrickWood lined out for you, you will not have to tear off the top of the oven and start all over again. And since the only point where mortar will need time to cure is after the bricks across the lip are installed, this will go relatively fast.
Small comfort, I know. At least you now know more than your know-it-all mason.