I used honey bee health formula in my early feed this year, not for mites, but simply for the health benefits...I also simply mixed thyme, spearmint and lemongrass oil onto the bottom board, and let the bees evaporate it around the hive. They made a roaring sound, it was a hot day, and they didn't leave the hive, but you could hear them evaporating the smell out with there wings. Essential oils are concentrated and would probably work better to get the benefits of the plant.
I agree to rely on them for any "solution" is folly, but if they help the bees immune system why not? Plants developed these oils over millions of years to kill bugs that would eat them, while honey bees work symbiotically with the plants so would they not develop an affinity for these oils? Obviously too much of anything is not good, but giving bees access to a herb garden or using oils in some capacity seems to use what nature intended.
An analogy I like is with humans, we can live without veggies, but are much healthier with them in our diet. With hauling bees to pollinate monocrops we limit their diet, how would we like to eat cornflakes for ten years straight? How healthy could we expect to be? Good on you for experimenting with dietary supplementation for your bees.