Thanks for reporting back! Unfortunately, java still has big problems using multiple versions of the same library, so your current approach is not a guarantee for success for future version updates (and even your current project may suffer from side-effects you have not encountered yet):
A good point of refence is running JanusGraph OLAP queries in the Gremlin Console using spark master = local[*], because an extensive test suite ran succesfully for each release. But practical projects, like yours, have other requirements than just served by Gremlin Conosole.
Best wishes, Marc