Minimum critical chance is additive with other sources of minimum critical chance (such as Demon Lord Marnox Card).
Minimum critical chance is an entirely separate roll to critically hit an enemy than regular critical chance. If you have a 50% minimum critical chance, that will be checked first; if it succeeds, you get a critical hit, and if it fails, then it will perform the regular critical chance roll based on your critical rate versus enemy critical resistance. By default, all characters have 0% minimum critical chance, but some skills have an implicit amount of it, such as Quick and Dead.
Additional Damage is reduced by an enemy’s “Additional Damage Resistance” stat and ignores defense. Reductions to this stat will increase Additional Damage dealt to enemies, but it cannot be made negative. I have been told that the Enchant Fire attribute may not function properly, though I haven’t confirmed this myself.
In general, Additional Damage (or property damage, as it used to be called) is only really useful in PVP, against monsters with extremely high defense (challenge mode stage 6/7), or on characters with items to effectively utilize it (the new Moringponia accessory set in KTOS and/or a Vaivora Sword).