1. If I have two IG Infantry squads (A & B). If B is behind A, and has to shoot through it in order to hit the enemy, the enemy unit gets a 4+ Cover Save?
If you fire through any unit, then the unit you are firing at will recieve a cover save, if more that 50% of the enemy is "in cover"
2. What if Unit A is spread out enough(2" between each model) to which all of squad B has clear LOS to the enemy unit, still grants enemy squad a 4+ save?
Firing through the gaps of a unit still count as firing through it. Although firing OVER a unit is fine, if you are elevated or otherwise significantly taller (such as a wraithlord firing over some guardians). Also even when firing through gaps, units such as monstrous creatures or vehicles still need to be 50% obscured
3. Above situation, does Squad B still get a cover save from squad A?
In most instances. Some exceptions include a barrage weapon hitting squad B (the shot counts as originating from the centre of the blast) or a nicely lined up shot that is positioned so it can see 50%+ of squad B
4. Area terrain is automatically considered difficult terrain?
Not automatically. Although in my
FLGS we assume all area terrain is difficult to simplify things you may want to play it as its not. You should discuss this before the game with your opponent, looking at each terrain feature and deciding what its properties are.
5. Does all difficult terrain grant a cover save?
No, same as above, you should discuss this, not all difficult terrain has to provide cover. I believer mine fields are difficult and dangerous, but they provide no cover at all.
6. If you have a unit in Area terrain (i.e. rubble of a building) and it is 3" inside the terrain does it give a cover save to the enemy unit?
Generally yes.
7. What if the unit is on a third floor (8" up), but against a window (Only 0.5" from edge of Area Terrain piece), does the enemy unit still get the 4+ Cover Save?
Firing through 0.5" i'd say no, although its hard to say without getting down and looking.