Slitherine

The rules say only one unit can occupy a building. I assume this is a village on the terrain lists.
Does this apply to a double village? I assume it does. Pretty big coverage for one unit.

Only artillery can fire against defended villages.

On the assault phase then, after everyone gets their 4 shots at them, potentially a lot of units could contact the village.
So do all these hits mount up to get the magic 6 to break the unit?

Does rear support count? Flank support doesn't seem in because this is treated as an obstacle.

How does the defending unit split dice if hit by multiple units?

There is a Buildings FAQ on the web page that covers all this.

The rules say only one unit can occupy a building. I assume this is a village on the terrain lists.
Does this apply to a double village? I assume it does. Pretty big coverage for one unit.

Two buildings that touch are classed as a village, or as you say a large village. One unit could occupy each building, so in this case you could put 2 units in the "village", 1 unit per building. The rule FAQ tells you how to move between adjacent buildings.