Sounds like a dysfunctional organization to be honest. Why couldn't you agree on the contents of the mobile API? If the backend team is slow, why can't the mobile team implement a proxy towards the backend to serve only the data they need?
I totally agree, it was a dysfunctional org, but sometimes fixing the org is extremely hard and you want to make progress today focusing on what you can do to improve the user's experience, instead of focusing on what others should change and do.
