I tried to answer that. Obvious failure on may part.
The written over verbal principle applies when there is a conflict. There is no conflict here. Rosenstein and Mueller (who entered into both verbal and written agreements) agree on the scope of Mueller's jurisdiction. There is no conflict, so we don't have to choose between verbal and written representations of the agreement.
Additionally, Rosenstein handed Mueller a stack of case files and said something to the effect that those cases were now Mueller's cases. That's unambiguous. Was that passing of files in writing? Probably, somewhere there is some sort of record of who has the case.
Looking at the question of written over verbal, that rule applies. I think your question is more along the lines of "if it isn;t writen, does Mueller have the jurisdiction?" Or "What is Mueller's jurisdiction." Again, see passing of case files, and maybe the Manafort EDVA case was handled by verbal agreement first, later backed with written confirmation.
I was wondering whether the verbal instructions were verbal for a reason(possibly expanded beyond the written) and while the parties were in agreement, one or the other might want to disavow the verbal instructions when/if brought out in the open.
As to whether Mueller’s jurisdiction is defined only by what is in writing, or could legally be expanded verbally, I didn’t have a clue. I did think that it would make sense to follow the same rule as Contract Law though.
It seems that the answer is not a yes or no, but it depends. Thanks for the explanations.