Having raised the issue of crewing and with the added comments it would seem that any significant delay at the diversion airport could become an operational nightmare unless a spare crew available somehow . A flight from Sin or Hkg is 12-14 hours depending on time of year so the duty time is already pretty much taken up and I imagine a delay of more than 4 hours means 18 or so hours on duty and no Captains discretion ? And then the crew on the plane have been on duty for say 17 hours and have to have another 17 hours rest as a minimum.
A lot of good reasons to try and avoid diverting if at all possible I guess