The part I dont understand is if a METAR which is a report issued every 30 or 60 mins, is issued with a TTF, which in AUS is valid for 3 hours, what happens to the validity of that same TTF when the metar is updated in 30 mins time?
As any forcast - if it is superceded by a newer forecast, the newer forcast takes precedence. The reason for the overlap is to allow an aircraft to get the forecast before take-off, and for it to last long enough to be useful for the time of landing.
Trends in a METAR may only be included... that if there is no trend statement, then the METAR should be considered valid for one hour only.
NO - a METAR without a trend is simply on observation of conditions at a particular time, and has NO validity period.