As for the other part of the discussion, critical hits do not exist.

There are two mechanics that people often call "critical hits":

Backstab - when you hit an enemy that does not have a red exclamation mark on top of his head, you deal damage multiplied by your weapon's Backstab multiplier. Knives do 6x damage, most other weapons do 3x damage, and there are few exceptions that have lower Backstab mult.

Staggered hit - when you hit an enemy while they are staggered, they take double damage. Enemies get staggered from taking Physical and Lightning damage and from having an attack Parried by the player.
PS: Every time you hear the "ching" sound on attack, that was a staggered hit.

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Increase in DoT damage comes from the simple fact that your damage is not a fixed number (ie. 120), but a range between X-Y, so that's why you will see a variance in the numbers. Moreover, Staff of Fireball deals Blunt damage on impact, which can Stagger the enemy, which makes your next Fireball deal double damage.