Question about Spark...

Hi, I'm confuse about Spark.
1) First of all the damage of spark at lvl 18 is 26–496, which means EACH spark projectiles can deal roughly 496 damage. However, there is something called 50% damage effectiveness > 1 projectiles = 13-496 damage, am i right?
2) Secondly, I'm trying to build a full ES with clarity aura, high crit chance + multiplier + cast speed. So, should i main spark or arc? According to a lots of people, spark is the strongest single target spell and will deal more damage if you can hit all of the projectiles, is this true?
3) Thirdly, which 5 support gems should i use to maximize the damage with spark IN PVP. My strategy is to meet my opponents head on with tempest and shotgun them with spark. I think ill go with lesser, greater multiple projectiles, increased critical damage, critical strikes, and 1 more. Should i go with increase casting, life leech (with ghost reaver ofc), faster projectiles or Lightning Penetration, opinions? (Fork is bad imo for the projectiles it created will each travel at a 60 degree angle from the projectile's original path and will less likely hit the original target, and noted that i will have critical weakness curse + conductivity active all the time with passive)
4) Finally, I don't understand the 30% and 50% less projectile damage from the projectiles multiplier gems. Don't these qualities make crit chance + crit damage gems look bad due to the damage reduction. Does this mean that ill have more damage with lvl 17 and up with lesser projectiles? What about greater projectiles, 50% damage reduction seems like a lot...
Ps: Thanks for taking your time to read + answer this post. And sorry for my grammar (rl wish that English is my native language) :D

Bump For Help...

Please guys. Just answer 1 or 2 questions.

1. Damage effctiveness of 50% means that any damage you would do after all other modifiers is multiplied by .5
2.
3. I think lesser+fork would be more dps than lesser+multiple, as you will end up with the more projectiles (5 initial +10 forked=15 vs 9 initial) and it has less projectile damage penalties.
It also seems unlikely that you will hit a target with a high percentage of the projectiles if you use both lesser and multiple. Faster projectiles make them go faster, do more damage, and last longer so I think it would be a good choice, also quality spark is helpful.
4. These mean that your projectiles will suffer from a .7 and .5 damage multiplier, which will lower their damage. It is a lot, which is why I don't think lmp and gmp are a great combo on spark.

Thanks a lot XD