Nice theory, except that the islamic declaration of faith (shahada) literally begins with “There is no god but allah & mohammed is his messenger.”
Not to mention, christians do not have any history of using the phrase “there is no god but god.” It is unique to islam. I would bet a ton of money that NO christian anywhere and especially in the middle east, would dare to utilize the first line of the shahada, in arabic, to proudly proclaim their christianity.
I know that, but if he doesn't have the "and Mohammad is his messenger" it doesn't matter. And we don't know if it doesn't have that part on it. Until and unless we do, we need to drop it.