WI: Charles Stuart (OTL Charles I) Died Shortly After Henry Frederick Stuart

It was for Frederick. His father, also Frederick, had a plan to put Palatinate at the centre of a Protestant network and therefore he married his children off to Protestant families. 

James wanted a French match for his daughter, Louis XIII nonetheless, but his advisors 'advised' against that idea, so Count Frederick it was. I understand her mother, Anne of Denmark, was not impressed and thought that her daughter was marrying down. 

If Frederick and Elizabeth hold Palatinate then there is no issue. The problem arises from the fact that Frederick lost Palatinate - James wanted it back and was prepared to fend off the Spanish by proposing a marriage between Charles and the Infanta and when that failed he resorted to military action. That too failed as Louis XIII refused to play ball.