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It is true nether essence requires no critical for life leeching, however it is affected by defense a lot. Even normally armored enemies such as simple captain ranked NPCs effectively reduce your healt leeching to 0. Towers in the other hand has no defense thus you leech HP with normal hit.
Critical nether hits grants regeneration over time with the given percentage of the damage caused by the actual hit and do not stack, much like other essence's effect. Critical hits (I guess this is true for other essences too) ignore defense thus do incredible damage.
Acid essence and offensive nox: the "sick" effect decrease healing with 20-80% depending on the level but not disable it. Max level offensive nox and acid criticals however disable hasty repair though you still can heal and repair yourself with other methods.
There is an usual misunderstanding about defiance though I didn't see you mentioning the defiance skill. Defiance skill increase your hull temporarily and not affect your defense against damage. Defiance do not affect aura buffs thus only your core HP will be increased and the bonus hulls from other skills will be just added on the top of that. Defiance itself also a buff so it will not affect your damage while being stationary with defensive spec, and don't make your hull buffing aura more effective.
Also aura affects your own ship too.