In instances like these, I would always recommend doing an equivalent degree at another university and then externally transferring. As you said, there is a risk of failing to hold an adequate WAM to transfer.
Also remember that graduation from MQ with Commerce rather than UNSW may affect your employability at graduation if you choose to do absolutely nothing during your degree. It's simply not a risk worth taking by doing Arts and hoping, especially considering you seem to hold Arts in contempt.
Lastly, commerce units done at MQ are more likely going to be creditable for a first year at UNSW so you won't have to do many or if any first year units at UNSW.