n00bie to FT here

I'm recently booked on UA's JFK-LAX-SYD-MEL outbound and MEL-SYD-SFO-JFK inbound. I have 2 questions :

1. Do I clear immigration/customs at SYD or MEL? Do I stay on the plane or need to re-board after they clean it ?

2. I've used miles for instant R-upgrade to J on the outbound, but inbound is R0 all the way. After hearing all the SHARES problems, is it better to just apply the miles to the inbound and waitlist, or use ExpertFlyer alerts for R inventory ?

Thanks