Is the information given here for the difference between Olyroos and Socceroos reliable?

Yes. The information given here which highlights the key differences and records of both teams is 100% reliable.

Are any of the key differences mentioned here likely to change?

It is unlikely that the key differences between the teams will change in the near future. And that's because the differences highlighted have remained intact for several decades already.

Who is the best player that played on both teams?

The best player to compete for both teams is arguably Brett Emerton. And this is a major reason why he gained so many caps for both the Olyroos and Socceroos.