yes all changes will be written to that specific ini file.
adding games is easy, if you hold shift in the scummvm menu you can bulk import, so point it at the folder where you store your games and it will scan the underlying folders recursivly. it will add everything you have stored there.
btw, if you are using retroarch, I ended up renaming all the .epicfail file extensions to .scummvm, the epicfail files were not taken for some reason. I used powershell to make it easier.
from the root folder where you store your games, open a powershell command box and feed it the following command.
Get-ChildItem -Filter "*epicfail*" -Recurse | Rename-Item -NewName {$_.name -replace 'epicfail','scummvm' } -verbose