One thing that complicates matters somewhat (as if they were not already complicated) is at the decision point marked isRoundtrip? in the fourth (penultimate) d

One thing that complicates matters somewhat (as if they were not already complicated) is at the decision point marked isRoundtrip? in the fourth (penultimate) diagram, where the affirmative case is handled within the new system. Given, however, what is being posited -- a legacy system that is not modular and which contains unrefactorable pathological dependencies -- the old system must also handle this case in parallel, in order to be in the correct state to handle future requests of a type that still need to be delegated to the old system. This parallel implementation may have to persist well into the replacement process, and the requirement for it to do so may mean that you still have to do double implementation of features and fixes for most of the transition.