1. A target number is what you need to reach on a D6. When it says +1 to the target number that's the number it is referring to. So if the attacker would normally need to get a 4, but their target is in cover, they'd need to get a 5 instead. They still roll the usual number of D6 to try and get that 5.
When the rules want you to add another dice they say '+1D6'.
Edited to add: For Defense, because the victim is having that +1 added directly to their Defense stat, they would indeed get an extra D6. Realised afterwards this may have been your actual question sorry!
2. I think this is referring to a model somewhere else who is shooting at a model who has another model in its personal space. So if Bill and Dan are in melee combat, in each others' personal space, and Dusty wants to shoot at Dan she must add +1 to her target number.
If it's the rule I think it is, the longhand version under Ranged Combat in the main rules says: "If your target has any other models (friend or foe) within its personal space, add +1 to your to hit target number as it will be a little more difficult to hit your intended target."
Hope that helps :-)