the Plain Java Client Libraries contain pretty much everything that is needed to do the job. Just because you use an API does not mean you do not have to understand how the fundamental mechanisms work. Quite the opposite, the rules hold true to the API as well.  In general you create change sets, and check in the changes. You deliver change sets which completes them and sends them to the stream. You can delay delivering change sets, but if you want to do another modification in a new change set to a file that has changes in an open, not completed change sets, you would get an error similar to the one you mention. Try the sequence in the tool, before trying to use the API.

Some of the use cases have been implemented in this tool: https://github.com/jazz-community/ewm-scm-utils