How to enable/disable apps using ADB (Android 8.x)?
Q1: Oddly, this list doesn't seem to include the apps I really disabled in the Apps section:
  
  
  Briefing
  Google Play Movies &amp; TV
  
  
  Why is that?


Your output already listed the two apps. com.google.android.videos for Google Play Movies and flipboard.boxer.app for Briefing app. 

Optional: see my answer to know how to find an app's package name easily.

For your other issue:

This seems to be a problem in your ROM per se. My experiences with stock Android ROMs on Nexus 6 device and a near stock Android ROM on OnePlus 6 (Android 8.1.0) tell me this shouldn't happen. 

Normally, the apps disabled using shell user are not listed in Settings app, but any app disabled by any means are listed in pm list packages -d or cmd package list packages -d.

Disabling and enabling an app without root access is simple. Run:

adb shell pm disable-user PACKAGE_NAME  # disables the app for the current user
adb shell pm enable PACKAGE_NAME        # enables the app for the user, regardless of how the app was disabled.