It's possible to do so, yes. You should check with customs and Immigration before leaving the country to identify any potential problems... it's a lot easier to get them straightened out when you're still in the country rather than when you're stuck in an airport (or something) trying to reenter the country.
yes
Australian female citizen marrying US male citizen - which would be easiest marrying in US and come back to Australia to live or marry in Australia to live?
By naturalization or by marrying a Montenegrin citizen, but there is no birthright citizenship!
No. You do not become a US citizen by marrying one. That is for you to earn by your own abilities!
yes
Marry In Aus.
No. Marrying a US citizen does not make someone a US citizen.
The US passport is what you need to get back in. Unless you're already outside the US, I wouldn't leave until you have this important document. Technically, you could just reenter the US as a Canadian tourist with a Canadian passport under the VISA Waiver programm but why chance it?
no
Nobody, not even a Venezuelan can become a US Citizen just by marrying a US Citizen. But it is by far the easiest ways of getting the citizenship. A person marrying a US citizen will be first granted with the conditional permanent resident status. After being so for 3 years, they can file in their citizenship application, if they satisfy all other eligibility conditions.
No.
NO. It is not a criminal act.