So, a couple of questions:

1. Has the XML version (to read the PDG web info directly) been written? 

Yes it has been written, and there is a XML format defined, but it is not yet in use with the JAS code, although an interface does exist. There is documentation on the XML format at http://yappi.freehep.org. It should easy to generate the XML from the phythia common block, and it would not involve learning much about XML, just copying the format of one of the existing XML files, such as:

http://java.freehep.org/servlet/fhcvs/show/freehep/hep/physics/yappi/xml/2000/PDG-Properties.xml/1.6

What you have done sounds fine for now, so there is no hurry to make the XML file.

2. Should I provide a PYTHIA derived DefaultParticlePropertyProvider or a new ParticlePropertyProvider? If so where and how?  

I think it would be better to provide a separate PythiaParticlePropertyProvider, since it does not make sense to tie the DefaultParticlePropertyProvider in JAS to Pythia. It is easy to use your own PPP, just do:

ParticleProperties.setParticlePropertyProvider(new PythiaParticlePropertyProvider());

This registers your new PPP as the default for the current process.

 3. What should I do with the INTERACT lines?