Hmmm...I don't know how I feel about giving you too many of these answers. Four and five are really just factual questions you are asking, so I will let you figure those out. One, two, and three are a bit trickier, so I don't mind giving you hand (and I don't mean clapping.)
First, "Why do the Gods need each other?" The answer to that is pretty similar to why humans need each other. The Gods are sort of like teachers who have been assigned to different subjects. Each one is a "specialist" in some aspect of nature or whatever. For example, a God like Poseidon is the "king of the sea." In this way, the work is split up. These Greek Gods do not seem to be Omnipotent, meaning that they can't just snap their fingers and do "anything." There are some limits, it would appear, or self-imposed limits because they are afraid of stepping on each others toes.
Also, in a lot of cases the Gods are related, so they need each other in the way that a family needs other members...for the occasional picnic, support, and help. Greek Gods are more like "supermen (and women)" than we tend to think of in, say, the Christian God.
Mostly, I think the Gods need each other for entertainment. They are always screwing around with each other and getting into mischief. If it was Zeus by himself I think he would get bored of watching humans all day without "equals" to interact with.