Is Void Melee damage inherently better? Unfortunately, I don’t have items to test this.
Nothing in LE double-dips.
Generally more specific stats always have higher numeric value.
Melee void damage is more likely a flat stat (gets multiplied by the skill’s added damage effectiveness then added to the base damage in the damage calc) rather than a % increased modifier (gets added in with all the other relevant % increased modifiers then applied), the two are very different.
You might want to read the in-game game guide, it’s not perfect but it’s a good start.