Hi Andrew, (Aduv...)
Did you end up going down the custom token approach in the end? This is the way I have managed to get some integration working with LinkedIn.
What I do find confusing though and this is where maybe some Google feedback can help, is that LinkedIn IS actually there in the list of Provider Identities. I am not entirely sure why or what other purpose it serves, especially if you can't actually log in this way like you can with Google, FB, github etc. Has your experience been able to tell you why LinkedIn is shown here in the list?
We ultimately create our users in Firebase when they do a Social Provider Login for Google and LinkedIn, but for linkedIn we have to use the custom token approach.
Best regards,
Bhavin