It is possible to have multiple Master Player accounts with no associated title player accounts by deleting all of the player's title accounts, but not deleting the master account. In this situation, it is not possible to re-create a title account with the same email address, because the master account still exists with no related title accounts.
To fix this, two things are needed:
1. The ability to get a list of all master player accounts for a studio
2. The ability to get the number of active title accounts linked to a master account. This seems to be different from GetPlayedTitles, because even if the player's title accounts have all been deleted, GetPlayedTitles will still show the titles that the player had accounts in previously.
I hope this feature request is clear. If not, please let me know how I can clarify. Thanks!