thank you for your answer, which helped me to get into it. At the end I was able to achieve my goal with the webjar after all. I had to change camunda-webapp-webjar structure a bit (which I already found as a issue in JIRA) so I created my first camunda pull request (hope it goes well). For the webapp to be aware of my embedded process engine I had to add
\WEB-INF\classes\META-INF\services\org.camunda.bpm.engine.rest.spi.ProcessEngineProvider with a value
org.camunda.bpm.engine.rest.impl.application.ContainerManagedProcessEngineProvider and proper web.xml which I mostly copied from original webapps web.xml and only made a small change.
With this setup I am able to use my embedded process engine both in my custom JSF based app and using Camunda webapp at the same time.
Hope it will be helpful for others.