What makes you say they can survive bleaching? What dilution and for how long? Are you talking about plants that have tight crevices between roots and leaves?
Experience. I am very meticulous in my cleaning, yet I have had root mealies show up in a tank. I wash first in a solution of dish soap and water, giving a 10-30 minute soak there before rinsing well, then bleaching.
I use a solution of 10% bleach for 10 minutes.
No. They hitched a ride, I suspect, on a Sinningia, from which I'd even removed all the living leaves and roots and used only the corm. Root mealies are TINY, and seem to prefer tender plants like gesneriads and begonias. The reason I suspect the Sinningia is due to my mother plant being especially affected, and not having seen them in my collection until I acquired that plant.
I've been told that a dunk in rubbing alcohol is sufficient to kill them before placing them in a tank, but with some plants, I worry that adding that step to the process could be too desiccating. Root mealies have a lot of waxiness and can survive submerged for a significant length of time, much like scale, which is how they manage to survive this process.