Okay.
From your initial post, "By “screen expansion” I mean that the viewing rectangle (surrounded by black on my tv screen since the video doesn’t fill the entire screen)...."
This indicates that your settings are wrong somewhere; may be Encore, may be your TV or DVD/BD player settings.
Please post a screenshot of your Encore project settings and one of the Encore project panel with the timeline selected (so we see its properties at the top) including the DVD Transcode settings column to the right.