As per this bug comment, shortcuts for the keyboard shortcuts overlay get pulled dynamically from user's settings.
Is it possible to change which shortcuts are included (i.e. show more of them - Alt+F9 minimizes window on my computer but it is not included)?
All the shortcuts that show in the keyboard shortcut overlay are hardcoded. If you want to change or modify what shows up, you need to compile Unity from source.
In the Unity source, go to Plugins → UnityShell → src → unityshell.cpp and modify them as you wish and then build Unity from source.
My keyboard shortcut overlay modification to include the shortcut for Video lens:
I could patch unity as per https://bugs.launchpad.net/ubuntu/+source/unity/+bug/937334 or more specifically: https://launchpadlibrarian.net/94791764/videolensshortcut.diff