[HTGeek]

Have you tried following these instructions, from a fresh Kodi 18 install?
YOUTUBE PERSONAL API KEY INSTRUCTIONS by jmh2002

 Yes, I was wondering about that. But at the beginning it states:
 

NB: These instructions are meant for existing users with a working, signed in, and activated YouTube addon.
They are not meant to help you install, setup, sign in, or activate the YouTube addon itself if you are a first time user.

Please install, setup, sign in, activate and test the YouTube addon first, before creating your own Personal API Keys.

Which put me off trying it.  However, I'm prepared to try anything at this point, so I'll report back later when I've tried it, and from a clean Kodi setup.

Thanks. 

 Yeah, true. I'm not sure what he meant by that or why he added that statement. It's very difficult to get connected to YT using the developer API keys that come with the app. Quite frankly, it ought to just require users to create their own. This is a function of limitations set by Google. My point is that in order to actually sign in and activate the YouTube app, you will most likely have to go the API creation route. Anyone who gets in without that is just plain lucky.

I found jmh2002's guide was the only one that worked for me, mostly because his was the only guide I found that accurately describes the process, explains how you acquire and match up the 3 pieces of data required, and explains the registration process with YT that connects your Google account to your Kodi install, which is the last step to make the magic happen. I've also confirmed that restarting the box, YT connectivity still works. As long as you don't mess with the pairing process, it will retain the info on restart. If you wish to change the user, you will have to re-pair Kodi and an associated, existing Google account.