I am not sure if fateh originated at the time of first Amrit Sinchaar or if it was practiced before. It seems like it must have been introduced at the time of Amrit Sinchaar because Guru Sahib officially declared the takers of Khand Pahul to be Khalsa at that time. Since fateh contains the word "Khalsa", it must have originated at the time of first Amrit Sinchar.
I could be wrong. If someone has more information, please post it here.