What if I want the main subject to be outside one of the 5 AF bracket areas? Without the speedlight (in say P mode with spot metering) I could place the subject in the AF area, depress the shutter half way to engage focus and apply meter readings to exposure control, engage the auto exposure lock to freeze the shutter and f/stop values, and then recompose--however (from my understanding) with the speedlight in i-TTL mode the correct exposure is not calculated until the shutter release is pressed and the preflash fires moments before the main flash. Obviously there is no time to recompose.
Also, what if I am in manual focus mode and as such don't have the 5 AF bracket areas? What is considered the main subject then? The speedlight should know where the focal plane is, but what if I have both a black cube and a white cube on the focal plane? Which is considered the main subject?