Lightning Strike supports - Lightning Penetration or Melee Physical Damage?

lightning pen doesn't reduce resists I think, it reduces their max resists. So if a target had 10000% lightning resist, and capped at 75% resist, lightning pen level 1 will lower that targets max lightning resist to 59% for that hit. I don't think it reduces that actual resist.

Don't use LMP on LS, use Fork, it's way better...

Always use pierce with lightning strike, ALWAYS.

For some reason, only ONE of the projectiles from lightning strike can hit a single enemy "per attack".

Projectile that procs pierce, deals damage, but it does not count towards the "1 projectile cap", thus you can actually hit 5 projectiles with GMP if they all pierce.

Now, it's a good question if you should go with LMP instead of GMP, to deal higher damage per hit, to actually proc some shocks that would last a while...