Regarding intelligibility amongst Aboriginal languages - there were
probably no Aboriginal people in traditional Australia who spoke or
understood only one language. People from neighbouring languages
often spoke "in language" too each other and there was enough mutual
intelligibility to get by.
Secondly, an Aboriginal Song-line cut across perhaps as many as a
dozen different languages. A person who was custodian of a songline,
could "sing the song line" to a custodian of the same line living in
a different language altogether and be completely understood.
There was also a rapid borrowing of words between language groups
(due to the taboo against using the name of a person who had died).
This lead to a rapid linguistic evolution, making all languages very
heterodox and making the construction of philogenic trees (as with
PIE) almost impossible for Aboriginal Languages.