I tried a number of searches here trying to find an answer and came up blank, so I'm hoping perhaps some of you can help.

I live in the US (Austin, TX) and am exploring flight options for a family vacation to Australia/New Zealand in August for my family of four. I'm Platinum Pro on American Airlines, which translates to Oneworld Sapphire. I'm considering three airlines: American, Qantas, and United. I have no status on United, but they offer a non-stop between Houston and Sydney with a quite reasonable (imho) charge to upgrade from economy to premium economy (roughly USD $250/pp). American has the advantage of being the airline with which I have status and which would let me select "Main Cabin Extra" seats (halfway between regular Economy and Premium Economy), but they offer the least desirable flight options (two or three stops with a significant layover in LAX each way). Which brings me to Qantas...

Qantas offers non-stop service from Dallas to Sydney. Both Dallas and Houston are a ~3-hour drive from Austin, so reasonably convenient and much cheaper than flying out of Austin proper. The Qantas flight has roughly the same base cost as the United flight, but a much higher cost of upgrading to premium economy. According to this Qantas page, Oneworld Sapphire members flying on Qantas are eligible for "Access to preferred or pre-reserved seating." When I started to go through the booking process on Qantas, to test the waters, I could not find a way to indicate that I was Oneworld Sapphire (I entered my AA FF# but the website did not acknowledge that this meant I was Sapphire). As a result, I was not able to determine what seats I would be able to access as Sapphire, which makes it hard to compare against the other options (AA and United).

So (finally), my questions are:

• How/when during (or after) booking flights directly with Qantas does a non-Qantas Oneworld Sapphire member get access to selecting "preferred seats"? Do I need to book the tickets then call back (or return to the website) to take advantage of the Oneworld Sapphire benefit?
• What constitutes "preferred seats" on Qantas long-haul flights like DFW-SYD? Is it only the very small number of seats at the front of each section of Economy? Or Premium Economy? If the former, that's not very appealing. If the latter, that's very appealing.
• Any other advice you might have to offer for maximizing value-to-comfort for those flights? I'd love to be able to afford Business Class, but I'm not seeing any reasonably priced biz tickets or upgrades available (sigh).

Thank you!