There is no arrivals lounge at SYD. If you have a connecting flight you can use showers in the departure lounges. Otherwise most hotels are able to accomodate early arriving pax, it is a common request.

Best seat depends on your taste. If you want to be seated together, the only choices are the middle seats (no window and somewhat less privacy) or 1A/K which has some drawbacks as well as advantages. But most of the time higher status pax will snag 1A/K so your desire for them is academic. But these differences between seats are micro, and there is probably more difference between specific planes and flights than between seats. QF10-late is notorious for being a bad experience, so you picked the worst of the trans-pac flights in terms of that. Generally QF11/12 is the best, fwiw. Of course the worst QF is 10x better than the best UA or CO, etc.