#1 What does the Metadata Bank include?
High value metadata identified as consumed by more than one business unit.
#2 Can more fields be added?
The metadata bank includes only those fields that are part of the Public Media Enteprise Metadata Specification
If the field exists in the EMM Specification and it is needed by more than one unit, email metadata @ pbs . org to request the addition to the Bank. 
If the field does not exist in the EMM specification, complete a Metadata Proposal form which will be reviewed for specification inclusion by the EMM Stewards.
#3 Are the APIs backward compatible?
Not yet. When a new release is planned, an announcement is posted on the ChangeLog and sent directly to existing API token holders.   Announcements will identify if the release is disruptive or not.
When a release is disruptive, API token holders will be given a two week period to assess the planned release and to request a delayed rollout.
When a release is not disruptive, (a) an announcement will be made two weeks prior and (b) existing token holders will be invited to 'test drive' the new release.