If the Captain was uncomfortable continuing with faulty anti ice equipment for the next 9hrs then they would have made the call returning. The thing that would deem it safe to continue would be to guarantee that they wouldn't enter icing conditions. If not (and on a long haul flight it's never guaranteed) then a need to return would be necessary.
My guess for returning to MEL instead of SYD would be:
1. They took off from MEL, so returning to the departure port makes sense from a pax perspective.
2. They would have needed to dump fuel and can take up to 40mins to get to MLW, might as well use that time to fly back rather than hold over SYD.
In flight the use of anti ice (engines and wings) are only really used when necessary. For the 787 this is automatic. The ice detector will know when a build up of ice over a set parameter has occurred and will then automatically turn on the engine anti ice (EAI). The wings on the 787 are heated mats and don't use bleed air.
I'd only seen it turn on twice in 5 years on the 777.
