To meet all of your requirements, I think you're kind of stuck with American Buttercream. While Vancouver Canada isnt quite as hot as Australia, I've had great success this summer using the following recipe for over 1000 cupcakes this summer:
1 cup salted butter
1 cup shortening
1 kg icing sugar
2 tsp clear vanilla
Butter extract - I find this helps to reduce the sweetness
5-6 Tbsp high fat milk
This makes a nice smooth buttercream that is a little...