It's not much of an answer, but...
For the preloaded Modern UI apps on the start screen, they are "Appx provisioned packages". They can be manipulated using PowerShell (administrator access recommended). List them using:
Get-AppxProvisionedPackage -Online
Take note of the "Package Name" and remove them using:
Remove-AppxProvisionedPackage -Online -PackageName ...
For the Pokki start menu app:
I removed the Pokki start menu app by first running its uninstaller as admin from C:\Users\Default\AppData\Local\Pokki\Uninstall.exe
and then loading the default user's hive in regedit and removing any mentions of it there (didn't take notes for this).