November for most, but the birds will beat you to them if you don't stake them out or cover the bush. I don't know about the ones in Florida, they bloom much earlier like in winter, they bloom here...
It's as safe as any fuel provided the chimney draws appropriately and the wax Myrtle is well-seasoned, I.e. dry and not green.. That being said, it's a soft wood and burns quickly with little heat...