I've sprayed the entire plant and soaked the soil for mealybugs before with a 50% alcohol solution and had no issues, but then I've also killed one of my favorites by screwing up the ratios when mixing it.
Just saying.
Otherwise your gardenia looks almost as unhealthy as mine. Want to swap notes, find out what we're doing wrong?
The overall health of a plant plays a big part in how far any infestation or infection can go in a give period of time, so keep it as healthy as possible.