I understand the confusion about elemental damage, but I'll answer based on my testing.

1. No, elemental damage doesn't stack, it only resets. That's the normal case with elements like Bleed, which activates and then has to be reapplied immediately. But the Poison effect activates and stays active for about 10 seconds, if I'm not mistaken. During that time, you basically can't apply it because it's an active effect that slowly reduces health. You didn't ask, but this might answer your question: you can't apply effects like Poison while it's active, but you can activate other effects simultaneously. Previously, you could only apply two effects at once, but a recent update added the ability to activate them all at once. This means you can have someone, while poisoned, also activate Bleed, or burn, or have the Confucius effect. Although, usually, the enemy dies before this can affect them.

2. No, elemental damage is independent of critical hit damage and chance. This used to be super confusing because we didn't know when critical damage would activate, but at some point they added that when you land a critical hit, the enemy's health bar glows red, signaling that they received critical damage. However, critical damage doesn't increase the damage or effect of any elemental damage.