Doesn’t seem constitutional to me that the president would be required to appoint a deputy Attorney General who had accrued during his tenure many questionable disqualifications. That does not seem to comport with the president being the chief executive officer under the Constitution. His hands should not be tied. He should not be forced to appoint someone who is conflicted, perhaps even with grave legal concerns. That seems like the AG succesion protocol is the law that is unconstitutional. A chief executive should have flexibility to appoint a replacement that has no legal problems or other disqualifications.
That's not the issue.
The law that creates and funds the Department of Justice stipulates rules for vacancies.
It is possible - just barely - that this law requires the most senior Senate-confirmed official at DOJ (Rosenstein) to act as AG until Trump nominates, and the Senate confirms, a replacement.
The Court would not force Trump to appoint Rosenstein. The Court MIGHT declare that, under the law, Rosenstein is AUTOMATICALLY Acting AG.