I don’t think anyone knows definitively until the thing gets rolling but LiftBlog describes the system:
The longest of three individual lines at Walt Disney World will have two angle stations, one of which will serve the all-new Disney Riviera Resort opening in 2019. All three lines will meet at a hub on the south side of Caribbean Beach Resort, where guests can change cabins based on destination.
Based on that description of the flow, POP/AOA guests have to make a connection at CBR to get to/from either park. Boardwalk area guests will need to connect at CBR to get to DHS. CBR guests get non stops to/from both parks and Riviera guests get a non stop to Epcot and connect to DHS.
So as for the Riviera “mid line boarding” scenario I’m guessing with you that empty cars will need to be dispatched from the end points. Additionally CM involvement will be key to add folks to an non-full car coming through from either direction, of fill a car that was vacated. Based on the lift blog articles the turn station at Riviera is moving entry/exit only with no provision for stationary entry exit like at the end points (double turnarounds). That is an interesting piece and I wonder how they will handle it at Riviera - scooters etc.