Reaver, putting the brain in the jar makes them a cyborg. Whether they're inserted into a drone, or just left someplace to soak in the nutrient solution, they're a cyborg. That is what a cyborg is, a brain in a jar, which sometimes gets put into other things.
And yes, under normal circumstances dropping to .01 Essence would destroy your resonance abilities. But see my earlier comment about how MCT discovered some cyberware that makes that not happen. Or just read Corporate Intrigue. It'll be in the chapter where they rescue Puck.