When I booked my *A RTW last year I was told the rules ex-UK prevent the use of any codeshare flights. The only exception was LH codes on BD as they were slow to get the booking class in place when they first joined *A.
WRT backtrack. Our original plan had us flying LHR-SIN (stopover), SIN-DPS (stopover), DPS-SIN-SYD. This was allowed under the rules but we would not have been allowed a second stopover in SIN. Eventually we went SIN-DPS-MEL-BNE-CNS-ASP (ground) AYQ-SYD. This meant a lot of ziz-zag and (in theory) a bit of back tracking but it appeared to be within the rules.
Full details of our trip are at:-
http://www.flyertalk.com/forum/Forum81/HTML/001425.html