You're correct that openni_tracker is incompatible with openni2_launch. The issue comes down to the Openni middleware NiTE. The openni_tracker uses NiTE v1.5 (I'm not sure if earlier versions of NiTE work, as I've only used 1.5). NiTE v1.5 is still available at various places (for instance, on the CD I received with a recently purchased Asus XtionPRO Live).
However, NiTE v2 is required for skeleton tracking with openni2. My understanding is that Apple bought the company that was developing NiTE and subsequently "closed-sourced" it -- that is, pulled the distribution licences. This makes it difficult, to say the least, to find a copy of NiTE2. And if you do, it's likely the source is in violation of their license (again, this is just my understanding of the situation).
Now, if you can find a copy of the NiTE2 software, the openni2_tracker ROS package found at https://github.com/futureneer/openni2-tracker is functional, although it could use some tweaking.