Diablo – What does the Dark Cloud rune do

1. Depends on many things including your skills, class, magic affixes, etc.
2. Open your Inventory and click the Details button. It's under the Offense section, which is at the top.
3. Yes.

Yes, they are. The easiest way to check this is to find a weapon with a socket, since you can manually add +min and +max enchantements to them. Now add a Ruby to it (enhances damage by a certain range). You will see the DPS go up in the item tooltip.

Also, it worked the same way in Diablo 2, so it's likely that they simply just left it the way it was.