I am going to try to explain my ideas for the dynamic assignment system. But to be super clear: the most important aspect of this design is that it preserves all the current order system elements as they exist in 1.13 and it is entirely opt-in. If you never flag stops as allowing schedule changes, and keep the schedule shift train sets as a single train, never repeated, you get Schedules 1.0 and your trains never dynamically change schedules.
How is a train assigned to a shift? Let's think about the easy case: a train has been just purchased, or has just been intervened. In this case the train is empty of pax and it has no location on the map, therefore it is valid to be assigned to anything. If this train is then put in the allowed set of trains of some Schedule Shift, and that shift becomes available, the sim can assign the train to the shift. Its state will then change to running a shift, the schedule system will record that particular shift as being take