I'm glad you asked! I hadn't given it serious thought until you did.

Small correction to my chronology: I'd forgotten to check the birth dates of FW2's sons, and sure enough, he had a couple small children in line for the throne in 1774 (birthdates 1770 and 1773). But since Heinrich's going to be regent in the absence of their father, and (at least in Fritz's mind in 1776) Cardinal Richelieu for their father, the same point applies.