Re Q1: You are not able to booked mixed class on the way to Oz or return.
So LHR-SIN in WT+ and SIN-SYD in J is not permitted.
A few years back I booked J on the UK to Asia sectors and F from Asia to Oz and same in reverse and the computer couldn't handle the calculation and so we got the booking for 200k Miles and a 241 ^^
Last year when I booked a ticket they would not permit a mix of class on the 2 legs of the trip to Oz (even though there were 3 day stopovers in SYD/BKK). So I had to book F out and J back for 250k miles.
Of course it could be one of thos "Hang up and try again" scenarios!!