What I'll do next is:

• Load this .json file and actually use the offset as a delay when loading the song so it affects gameplay (this could eventually be hidden behind a setting, but I don't know how to do that yet)
• Move some code to new files, as I don't want to clutter ScoreScreenMenu.cs
  Now works for CON and SNG packages as well :)

I was going to reply to your question from the other PR about where to save the JSON file, but it seems you have it well in hand. Normally things go in a subdirectory, but there's not really any place it obviously fits, so where you have it now is fine.

As for how to add a toggle setting, the general idea is to define the settings in SettingsManager.Settings.cs, then expose it to the user in SettingsManager.cs where ever in the DisplayedSettingsTabs list makes sense for it to live. You can then access the value of the setting with something like SettingsManager.Settings.YourSettingName.Value.

I hate adding other stuff that you'll have to figure out, but it would probably be best to make the button require a long press to activate so people won't be as prone to hitting it accidentally. It's not the most helpful pointer since it is large and spread out over several files, but MusicLibraryMenu should have an example of how to make a navigation button require a long press in there somewhere.

This still requires just a bit of testing, but if anyone could test this with more than one player it would be great. Appart from that everything should be done. (Tested, works fine except when playing online because of the delay.)

edit: Thank you wyrdough for your help with finding the necessary files, everything was easy to adapt :)

edit: Tested with more than one player and it works perfectly

The last bug that I see is that YargLogger.LogFormatInfo doesn't seem to make anything appear, but that might just be on my side. Anyway, I don't think it came from my code.

Also when doing a setlist, buttons won't fit on screen :(

Also really important to do when Online gets added to the game (conclusions after testing with the online alpha from pythonology): Only consider local players for the average, or else some horrible delays are considered from the online delay. I added a comment for this and tested it with the onlive dev build available, works perfectly.