As long as the segments up to and including your first international/intercontinental flights are confirmed, you can keep the dates of the rest of the segments open. (Bit pointless as you are allowed free date/time changes at any time anyway).
Rerouting mid trip is also permitted, however there will be a reissue fee for any voluntary ticket reissues.

No restrictions on routing, apart from the city stopovers/transits rule, origin country dept/arrival rule and the 1 only crossing of both Nth Pac and Sth Pac, as well as incorporating both AU/NZ and US/CA in one trip.
Your mileage limits will affect any plans to do massive backtracks like HKG-AKL-PER-AKL-SYD etc more than anything else.

As far as 2 cabin aircraft go, you'll find if there are 2 cabins on the international flights they will be Business and Economy anyway, so that's a moot point. None of the :-: partners run just a First and Economy on long haul Internationals that I can think of.