How is the phrase "In sha' Allah" (God willing) used in the context of matters that have not yet occurred, despite the fact that what God willed has happened and what He did not will has not happened?

Both existence and non-existence are by the will of Allah Almighty; whatever Allah wills comes to be, and whatever He does not will does not come to be. This applies universally to what He Himself does and to what His servants do.

What exists, came into existence by His will, and what does not exist, did not come into existence by His will. Therefore, it is permissible to attribute the non-occurrence of an action to the will of Allah, such as saying: "I will not do such and such, if Allah wills," meaning, if Allah wills that I do not do it.