It's hard to predict how often Taunt will be used at the actual VGCs. It seems that on PO, it was very common at first, but then once people started using Mental Herb it's use dropped off sharply. It's not used too much on there now. However, from conversations with other people on other forums, Taunt still seems popular. If your team absolutely lives or dies on whatever setup, you probably should use Mental Herb.
I honestly don't think I've ever seen a Chandelure with Flamethrower. They usually use Heatwave/Overheat. I have seen a few Fire Jewel ones, but not that often. If they seriously want to kill Whimsicott they will.