Question about barrage

I'm running an elemental wander Scion with Barrage as a single target damage skill. I got a Shroud of the Lightless earlier today and I need to rework my gem set up. Since the Shroud has Lvl 20 Elemental Penetration I can get rid of Lightning Penetration. However, I have a few questions before I make any changes.
1. How does penetration stack if I were to use Lightning Penetration in addition to the Shroud's elemental penetration? I'm assuming the Shroud offers 37% penetration just like lvl 20 Lightning Penetration.
2. Is LMP a legitimate option with barrage or does the damage penalty outweigh the added projectile benefit. Also, does the damage penalty only affect my projectile damage (which would only be the physical damage I assume?) or the elemental damage attached to the projectiles, too?
3. What would be my best option as far as a 5-link set up for Barrage then?

1 - It stcks additively, meaning all elemental damage gets 37% penetration and then lightning gets an additional 37%.
2 - No as far as I know - just run GMP instead of LMP until you get a Dying sun. In that case (I never used it tho) people say it's better to replace it with, let's say, sower projectiles for that t h i c c more multiplier because you already get projectile(s) from the flask. It's very easy to check in PoB anyway.
3 - Again, testing it yourself is usually the best because you are saved from biased fanboys, but generally speaking, you usually use Barrage, Added Lightning, WED, GMP and the last one I'm not sure about, but I used Added Cold in Abyss, but if you'll be pure lightning, then maybe the pen works better.

Hope I helped.