In most of the cases, crit is rolled once per activation. So if you use Split arrow, either all the arrows crit or none of them crit.

Obviously because we can't have it too simple, there are couple exceptions:

Some skills roll crits different (Orb of storms rolls per lightning bolt, Balde vortex per hit, Blade flurry per hit. Note that "per hit" means that all enemies still either get crit or not) and if there are modifiers for "increased chance to take critical strike" on enemies such as Assassin's mark, it can cause 1 mob to be crit but the other to be hit.

Do ailments stack or do they reset or do they take latest or take most damage?

Currently Poison is the only soon-to-be ailment that stacks by default. You can have as many instance of it up on the enemy as you can get.

Ignite, however acts a bit different. Only 1 instance will deal damage to the enemy simultaneously but multiple instances can be "on" the enemy (they're tracked by the game but irrelevant to players). Ignite always deals damage based on the HIGHEST dps instance there is.

Bleed acts like ignite.