We are all in agreement the worst case scenario is clearly RWY30 WET @ 30 degrees, but let's break that down and compare with VA's OOL-PER, which was introduced prior to the runway extension at OOL and did require seats to be blocked out. HBA is the driest capital city in the country, so the wet runway won't occur as often as in OOL (I'm trying to say this with a straight face as it's currently pi$$ing down in HBA and has been all weekend!). A 30 degree day will occur about twice per month from October to March, and never from April to September. Furthermore, with a 1930 take-off time, the temperature will have dropped well below 30 degrees for the Monday and Wednesday flights, but may occur for the Friday 1445 flight. Even then, with a 75 tonne TOW (which includes the 4T penalty for RWY30 due terrain limitations to the north), it can make it. This assumes a full load of pax; make it a typical 80 per cent load and you get an extra 3T or so to play with. All in all, I'd say once per year at most the flight may have to divert to ADL or KGI (otherwise ditch the bags and send them separately via MEL), but this is no different to a number of recent tanscon BNE-PER flights that diverted to MEL or ADL.
With regards to the success of ADL-HBA, JQ's exit was in 2006, well before MONA was built and the recent surge in passengers which has made HBA consistently the fastest growing airport in the country. The VA exit in 2009 was a month after TT began the route with 319s four times per week in competition, and occurred simultaneously with VA dropping 3 daily MEL-ADL services, one of which was the aircraft that operated MEL-ADL-HBA-ADL-MEL. You don't operate a route with a daily 73H for five years if it's not viable; you either cut it after two years or reduce frequency to a few per week. It would be an indictment on the JQ leadership if the recently reintroduced three per week ADL-HBA was not increased to daily for NW18-19, especially given it operates on arguably the three lowest demand days of the week, and the aircraft sits idle in ADL on the other days.
HBA-PER will have about 50,000 seats per year at three return services per week. Tourism figures from TAS quote 50,000 annual isitors from WA; figures from WA quote 40,000 annual visitors from TAS. That makes 90,000 tourists, which is 180,000 return seats, not to mention increasing the market due to the nonstop service which cuts 2-3 hours off the trip each way, and not to mention any business or mining travel. VA's HBA-PER will work; the Government needs it to so as to justify the $40M runway extension given the current lack of interest in direct flights to Asia.