Question from a developer:
If a player does not have any linked accounts remaining, is there any way to still login as the player to set up another linked account?
All the logins take some form of credential, each of which can be unlinked from the account. If you've unlinked everything from that account, you no longer have valid credentials you can use to sign into it. There is no "backdoor" that would allow you to sign in as that player, as that would be a security hole.
However, there is a customer support story for this. If a player gets into this state, you (or your customer service reps) can get them back to their account as long as you have the LoginWithEmailAddress API call enabled somewhere - either in your game, or on your site. In the Players tab, once you've selected the player in question, you can unlock the email address and enter a valid email for the player. Once you've saved that, click on the send password reset option and save again. That will generate an automatic email to the player with a link she can use to set a password. Those credentials can then be used with the LoginWithEmailAddress call to log into the account.
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