OK, I can help with some of what is being discussed in this post. I completed this trip last week.
I was looking for points flights from Europe back to Melbourne and the only availability was on QR988 from Doha to Adelaide with a 'technical' stop in Melbourne. I checked in from Europe and asked them to tag my luggage to Melbourne. They weren't able to do this (their system didn't allow this with the way my flight was ticketed). Boarding pass and bag tag printed with DOH -> ADL and no mention of Melbourne.
Flight arrives in Melbourne just before midnight. I get off the plane and there are Qatar ground staff at the end of the aerobridge with signs directing Adelaide transit passengers off through a transit area (ADL transit passengers need to remain in the terminal). I tell the ground staff that I am not continuing on to Adelaide and I have a checked bag. They write my name down and tell me that my bag will be offloaded. I follow off the rest of the people disembarking in Melbourne. Smartgates work a treat and I have no issues clearing immigration.
Not sure how long it might take for my bag to be recovered from the plane, I find the baggage services counter between immigration and customs. I explain to them that I have chosen to offload myself in Melbourne and that my bag was tagged to Adelaide. They take down my details and tell me that my best bet is to clear immigration and return the next day to collect my checked luggage. I clear customer with just my hand luggage and head for home. No questions from customs. I get a call around 10am telling me that my bags have been located and can be picked up. Menzies baggage services has a counter inside on the arrivals level at Melbourne International T2 (just behind the Skybus counter - it has two large black doors). I walk in pick up by bag and walk out again. The whole thing takes 5 minutes. I don't need to do anything with customs. You pickup the bag on the cleared / arrivals side of the terminal. Was a little painful to have to return to the airport to get my checked luggage as I live some distance away, but totally doable, and would be an absolute breeze with carry-on luggage only.