We did recently add special b
atching support for the User Permissions API which should quite dramatically increase the performance of adding users to accounts, properties and profiles, there are also quota incentives to use batching with the User Permissions API.
As for authenticating with your mcc account the quickest way to check what would be possible would be to authenticate with that email and call the
Account Summaries list method (try-it) and that would tell you what Google Analytics accounts the MCC has permissions to access.