The API should be as close to the API of react-native as possible. If there are custom functions or variables exposed in a mock that isn't in the real API should be prefixed with 2 undercores, __. Requirement of parameters should be handled in the same way, if a parameter is required in react-native, it should be required in react-native mock, if it's not needed, then it shouldn't be needed here either.
"leaf node" components can be mocked out as just components returning null in their render method, and tested at a higher level using shallow from enzyme. Compound RN components like ListView, NavigationExperimental, etc. can do more complex things.
react-native-mock should be compatible with the most recent version of react-native. Backwards compatibility is not something that needs to be considered, if the API changes, then so does the mock!
In simple: Don't. Leave that for us to manage when deploying!
When adding a new mock, make sure it's got some in-depth tests, and that the linter passes. PRs with failing tests or tests that aren't in-depth enough won't be merged. This keeps the mock as stable as possible.