re: point 1. Just because this is what they prefer, it doesn't mean a person is bound to follow their advice.
re: point 2. I'm not convinced this is a criminal offence under Cth laws (Crimes Act or Criminal Code). Note I'm not suggesting the mail be read, just opening, sliding in their own correspondence and sending it back to sender. Just a modified version of return to sender (NATA).
I think people say 'never open other people's mail' as it is normally the wrong the thing to do, and can result in criminal charges if material/goods/information is misused.
This situation with a person misusing a postal service because he is either lazy in changing address details or more likely harassing his ex, is not a normal situation, and failing to find a criminal charge likely to apply then the person receiving incorrectly addressed mail seems to be within their rights to do as I suggested. This behaviour by the ex may in itself be an offence.
Criminal Code:
471.12 Using a postal or similar service to menace, harass or cause offence
A person commits an offence if:
(a) the person uses a postal or similar service; and
(b) the person does so in a way (whether by the method of use or the content of a communication, or both) that reasonable persons would regard as being, in all the circumstances, menacing, harassing or offensive.
Penalty: Imprisonment for 2 years.