The use of YBHI would only work for one aircraft in an emergency. If two B737 aircaft diverted to YBHI there is a very good chance the first aircaft could damage the runway and close the airport. If you want to include YBHI you might as well throw YPAG and YMTG into the picture. But none can be planned as an alternate.
People are throwing up 'what/could/should' have been done, while sitting at home looking at back on information that has been gathered over the 4 weeks since the event. Have a look at the tracks shown on the map included in the ATSB report, both aircraft flew past Mildura, so the crews would have an idea of what the weather was like, then 15-20 mins later they are provided with YMIA weather saying it's ok, the 2300 METAR has the cloud higher at 3900'. When both crews made the call to divert they had minutes to decide based on the information they were given and on what they saw when the flew past YMiA.
If both crews had any indication of the cloud lowering or fog forming at YMIA, they would have continued to YPAD and busted minimal via an Autoland following an ILS approach and this thread wouldn't be here!
As I said yesterday, let's focus on the prevention on another similar event.
What I see needs to be done is;
1. Rather than spend their money on building car parks and shopping centers, Airports should forced to upgrade all ILS's to at least CAT2 ASAP and work torwards CAT 3.
2. CASA to regulate Flight Dispatcher training like they do with load controllers. In the USA flight dispatchers are includedd in the FAR's and are licensed and trained to FAA standards.
3. Complete review of the BOM and the impact on years of funding cutbacks have had. The quality of the forecasts has declined and it's not just the lack of fog forecasting, during the summer months lack of thunderstorms is also and issue.