You need 4 hits to apply 4 shock stacks, for each enemy individually. 100% stack chance.
I don’t know if the shock stacks count for both chars.
They wear off after some seconds.
Only shock damage.
Weaken doesn’t change the element, Effective does only upon damage calculation. If you use Prismatic/Ascendent/Elements stacks will only be applied if the attack is a shock attack. Effective doesn’t matter.
It should add +1 Shock Effect Max, but is bugged at moment and can currently not be activated. It will most likely be buffed in the future after being fixed as well.