Humanoid Slaying has already been determined to work for crits like this:
Base Damage + [Base Damage * (Crit Modifier * Slaying Modifier)] * Slaying Modifier
Thus with Mortal Shots and Humanoid Slaying, it would look like this:
Base Damage + [Base Damage * (1.30 * 1.03)] * 1.03
Just did some testing with a Fine Light Crossbow (Damage = 29-29) and Relentless Earthstorm.
Firing at the Void Conduit at Ultris (rough numbers):
301-302 Damage base
717-718 Damage on Crit
301.5 + (301.5 * 1.339) * 1.03 = 717.3
Tricky part now:
On Dr. Boom:
321-322 Damage base
791 Damage on Crit
Without the Relentless Earthstorm you'd get:
321.5 + (321.5 * 1.339) * 1.03 = 764.9, which is not 791.
However, what really happens in game (numbers fully represented):
321.5 + [(321.5 * 1.30 * (1.03 + 1.03)] * [1.03 + 1.03]
[top] 791.1
Mind you, that damage without Slaying would be:
321.5 + (321.5 * 1.30)
730.5
Taking out the +3% damage bonus you'd get:
321.5 / 1.03 = 312.1
312.1 + (312.1 * 1.30) = 717.83
This would mean that combined together, Humanoid Slaying and Relentless Earthstorm Diamond increase overall crit damage by 10%, compared to not having points in either talent (mind you, this factors in the 3% more base damage as well).