You can do the "lets all be Platinum" unchallenge more than once. You cannot use it to get back status just lost however

The upgrades can be converted at 4 upgrades = 2000 points

Don't sign up for both challenges. It has no benefit to him and may be baneful. SYD-LHR or MEL-LHR will complete the Platinum challenge on its own and he will get the 100% bonus for it too. Sign up for both and he may only get Gold from the flight to Oz and still need another 5000 points to complete the Platinum challenge. You are risking getting less points for nothing

Dave