If no skill point is invested, will only remain active while equipping an item supplying it as a skill bonus: removing the item will deactivate it, and it will remain deactivated until cast again, even if the item is equipped again.
but:
Skill level applied when cast, which affects base lightning damage, strike interval and duration. Lightning Mastery and other +% Lightning Skill Damage applies to strikes as they happen.
so: you need to invest at least 1 point or an item with 1 point to the skill. prebuffing then indeed grants higher lvl to the skill which does NOT need to ne maintained. however, skill dmg is calculated on hit, so only your current gear is considered in this regard