There is nothing in Lightning Vortex's text that says it cannot be negated, either negating the activation or negating the effect.
No, you can have 3 copies.
Horus the Black Flame Dragon LV6 will not be affected by the effect of Lightning Vortex or Gravekeeper's Servant.
Yes, Lightning Vortex destroys monsters, so Stardust Dragon can negate it. It doesn't matter that the number is variable, or that it does not target. To legally activate Lightning Vortex, it must be able to destroy at least one monster on this field, and this same condition is what allows Stardust Dragon to negate it too.
Lightning Vortex is non-targetting so it never really activated 'on' a specific monster, even if it's alone. If your opponent uses Lightning Vortex and you have an XYZ monster, that monster will be destroyed, as per usual, unless that particular XYZ can save itself from destruction with an effect.
For breaker they can, but not for dark hole or lightning vortex, because those two do not 'target'.
Lightning Vortex is not banned in any format. Neither is it limited nor semi-limited.
Lightning and wind makes a vortex because the wind creates a spiral figure like a tornado, while the lightning makes it able to travel through time or able to teleport anywhere in space, even in the sun. A vortex is A.K.A as the " tornado in space "
Yes it does. Lightning Vortex does not target. Blue-Eyes Shining Dragon's protective effect works only against cards that target, like Exiled Force.
Dark World Monsters have to be discarded 'by an effect' of a card. If you discard it as a cost, like that for Lightning Vortex, this will not trigger the Dark World card, and you will not get its effect.
One of them for sure is Lightning Vortex.
fissure, smashing ground, lightning vortex,
no it does not. It only has storm Pegasus and lightning l drago, which are not the real type