So if some paid advertisement contained defamatory material, would the publisher of the advertisement be held liable?
Yes. This is normally the case under defamation law in Australia. Three's a lot of established case law. With defamation it's standard practice to sue the publisher as well as the author because the publisher generally has deeper pockets. In this case the court decided that providing indexed hyperlink does not meet the legal definition of publication.
Should Google be directly paid by the defamer / slanderer to be held liable?
I think the distinction here is that automatic indexing does not meet the legal definition of publication because it's not intentional. Receiving payment would establish an intention to publish, but that does not preclude that a company could be liable for defamation if they intentionally published defamatory material without receiving payment.
What if Google benefited financially by promoting defamatory material all over the corners of the world?
I don't think a defamation victim will have a case in an Australian court if the defamation occurs in other countries. However previous case law has established that under Australian law a publisher can be liable for presenting defamatory material to readers in Australia even if the material is hosted on servers in other countries.