1. Yes wielding two weapons means you deal damage from both weapons.
2. Speed is the speed with which you and the monsters take turns. Most monsters have the same speed as you, which means it is alternatively your turn to do an action and then the monsters' turn. When slowed down, you're at half speed meaning you play a turn every two monsters' turn. Note that this is a simplistic explanation, the game uses a scheduler; which means any kind of interleaving is possible, but this is what happens most of the time.
3. Yes parry and shield stack.
4. Both parry and shields are modifiers to contact hit probability (although shields also modify ranged hit probability), i.e. they decrease likeliness of being hit.