1,2)  Outposts can expand your ZOC even further.  Upgrading said outposts with High Tower will expand it again.  As far as when the population gives a boost, no it's said in-game.  It just happens as your population increases.  As far as buildings go,  you'll see +1 ZOC on things like Monument, Town Hall, etc..

3) Must be 3 tiles apart, it won't let you build if you don't.  Box will be greyed out.

4) No and No.  Sometimes wandering monsters will wander in.  Other players may be pains and try to run scouts/pioneers across your territory.  Even if you say get the crap out or we're at war.  They're doing the same junk next turn.  Function? It's like your empire/kingdom's borders. One other thing, some lair monsters may or may not attack you once their lair becomes in the ZOC. 

5) doubt it.  Cities initially form a 3 x 3 box around the center, where the pioneer settled.  Outpost have larger radius, 2 tiles out from center.

A) Cause they want to limit it to a certain extent.

B ) They just are, makes the outpost do something.

C) The portion of the ZOC where the outpost is.  2 tile radius from the outpost. 

After playing around with FE a bit more, I've come up with a couple other features/benefits of ZoC (addressing point #4):

1. It gives you access to resources.

2. It pulls back the Fog of War so you can see what's going on within your areas of influence.

Given that info, knowing how and when it expands is really helpful. Currently I've really enjoyed when the ZoC for my city increases, but it just sorta happens, and it's not obvious it's related to population.

Please, can we get some info in the UI on this? And does anyone know the levels at which ZoC expands for population sizes?