Egor, jBPM can definitely support your case but it is much more capable than this and thus you see all these dependencies. Some of them are transitive dependencies due to various features require them but it does not have to mean you will need them too. For example CXF are used for web service invocation and are brought to the picture by jbpm-workitems. If you're not going to use web service feel free to exclude these dependencies which will make your end application much smaller and without not needed libraries.
Human task module is indeed complex but it has ist reason to be like this - WS HT specification and the features in brings such as life cycle of the task and its operations.
If you use CDI and REST you should definitely look at version 6 as it's built for CDI - complete human task module is redesigned to follow CDI approach. That might be much better fit for your case.