On the other hand, I don't see why you can't book it as seperate legs using the voucher (i.e. treating GRU as a stopover/connection). I don't believe this breaks any rules: it's okay on a basic reward and the voucher carries no specific exclusion. I'm sure people have stopped in SIN or BKK on the way to SYD for example.

I'd phone again and speak to someone else.