Omnislash doesn't actually crit, it just makes your hero invulnerable while jumping between units. Every jump deals a static amount of damage, but your hero can still attack normally while jumping, and his attacks can still crit.
Ember Spirit's ability actually transforms the hero into a different unit with a very low base attack cooldown and maximum attack speed, and then makes it move to and attack every unit in the area once.
Neither ability actually crits. If you want abilities that can crit you'll just need to trigger the damage. Detecting the unit's attack damage is a whole other problem.