Hello conlabz team,
Maybe "verification mode" was just a bad translation. I'm using the French interface for the Google Tag Manager and you get this when go to Container > Version > Dropdown menu in the top right hand corner that reads "Prévisualisation et débogage".
I didn't want to manually modify that theme code. If I wanted to do this, I wouldn't have looked for a plug-in in the first place. In my book, the plug-in should do the job of placing the code in the right spot.
I'm a WordPress beginner, so I don't know if the Pinboard theme "called the wp_footer function" as you say. Your king advice if it does or not would be kindly appreciated.
Good luck with your plug-in. I will look into the question again when I have more tags to track, but for the time being I simply decided to stay with the basic GA code, because it's the only one I need to track right now.