4. Where can I obtain the insecticide you have to pre-spray the cabin with prior to arrival?
Callington make a complete range of IATA/ICAO approved products, "one shot" "top of descent" etc. These are industry standard and would be available anywhere.
5. Weather. I was planning to leave Thailand late April, to avoid the beginning of the monsoon there, so would be arriving to Australia in early May, and returning in mid June. What is the weather generally like for flying in Australia during that period?
Generally during your proposed flight periods the weather is gin clear, however as always ensure you have appropriate preflight and enroute updates. Have a look at
Airservices
With 1000nm range, there are numerous routing options, however on your return beware of westerly jetsteam activity, generally south of 25°S.
, these can be up to 150kts.