I think what you are trying to do is relatively simple.
You have a notification task that needs to occur 14 days prior to the Start of FAT task. This should accomplish that.
Set a successor relationship between Notification Task and Start of FAT task. In that relationship, set a Lag value of 14 days. Then in the Notification Task, set the task to schedule As Late As Possible.
The Notification task technically needs no predecessor.
In this scenario, if the Start of FAT task shifts in either direction, the Notification task will automatically adjust accordingly and because it’s As Late As Possible with a Lag of 14 days, the Notification task should always appear 14 days prior to the Start of FAT task.