In conclusion,
It's not so much a question of whether BoB would have been lost without American help, it's more of a question would they have one was it not for the foreign pilots who made up a vast part of their air force, who were fighting for your British freedom, and their own.
I think not.
I'm also of the school of thinking that given only hurricanes, the collective hatred of the enemy and determination of members of the RAF, the LW would have been beaten anyway. I do not think the will of the British people would have been broken, not until the germans would have been marching in their tens of thousands through the streets of London.