Thank you all (og, Mwenenzi, and Dave Noble)! I very much appreciate the quick, clear responses! They clear up a lot of mysteries, especially with regard to what qualifies as "preferred seating" (pretty unappealing on QF) and the reason why I could not select seats (that policy is odd, Mwenenzi).
I'll take a look at the other options, especially an Air New Zealand triangle route (IAH-AKL-SYD-IAH), since we would want to visit NZ. SYD seems to be the cheapest option for Australia proper, but I'll definitely keep an eye on MEL and BNE.