I took another look at my problem with a clear head and started a new project.
It appears that my disfunction came into play by selecting my Timeline asset in the Project bin and manipulating the End Actions. Because I have a "Play All" button wherein everything plays beginning to end sequentially... as well as a Scene/Chapter Select option with Overrides... all Navigation & End Actions needed to be made from the Button level as apposed to the Timeline level... because it is the individual Button on a specific Menu that determines the End Action.
Once I selected my Button, I could add navigation and the disc behaved as expected. If I were to select a Timeline... navigation would go awry and behave unexpectedly. Reason being: I needed navigation specific to the Button.
Once I selected something else like a Timeline, I was no longer building specific Button navigation. I could not set up a Timeline End Action as the Timeline is used multiple times with different End Actions. After I saw the err of my ways, it worked flawlessly on the first attempt.
In short, I was confusing Encore.
(I hope I have not confused you all as well)
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Mandy... You too may be confusing Encore.. I would suggest trying:
After dragging your Timeline Chapter Markers to your Menu to make the Buttons. Select a Button on your Menu and do its navigation & End Actions from there and NOT from the Flowchart.