I'm no Guru (no pun intended), but I'll attempt to answer these as best I can.
First, you have to change the way you think of "God." I grew in a fundamentalist Christian religion so am deeply familiar with the God idea from that perspective. An all powerful being that is separate from us. Something outside our universe or reality that is omnipotent and omnipresent.
From the perspective of Polytheism... how best to explain this...
Let's take your computer for example. Everything you see on your screen is distinct, all programs perform their own function, but are all made of the same electricity that courses through your PC. Every bit of information is powered by the same source. It's all energy, but every bit performs it's own function, whether it be a pixel on your screen or a number going through the CPU. With that in mind...
1. Why must there be multiple Gods/Goddesses? What's the purpose for that? If the purpose for different Gods is for each of them to control different factors of nature/the world, why can't one God/Godess handle everything? Why do they have to break off portions?
There is only one, still, per say. This of course differs depending on the school of thought. Some believe we are all part of that energy and there is no other, others believe we are all part of that energy and there are others, but outside of our universe. Other "Gods" that have nothing to do with our realm.
2. Is there a God/Godess that is completely all-powerful? If so, why can't He/She handle things on their own?
Yes and No. Schools of thought differ. The energy that you are (matter is only condensed energy) is part of a larger whole. That "whole" is all powerful, but our perception of what an all powerful God should do differs from person to person. Person A thinks God should not allow suffering at all. Person B sees suffering as a growth opportunity. The consciousness that is experiencing the suffering (the "watcher" behind your eyes) is having an experience. Much like you would if you went to sleep and had a lucid dream where you fought and lost against a dragon. You are not harmed, but in the dream the experience was quite riveting.
3. Are these Gods/Godesses eternal? Can new deities/divine beings be created? If so, how?
The universe wastes nothing. When you die, everything you are will be returned somehow to everything else. We humans like to think we can be ourselves forever... You can continue to be John Smith and that's eternal life. You are in a way eternal in that all the energy that makes up your current configuration will never cease to be, it will just cease to be John Smith.
If you had a glass of water and take a single drop to put under a microscope and label it "James," then later put the drop back into the glass, was it really "James?" If you could tell it that it was "James" would this change what it was? If you tried to pull James back out of the glass, would it be "James?"
4. Are there no Gods/Godesses that are all-powerful? Why are they considered God/Godess then, if they are simply higher beings of existence?
Again, it's a paradigm shift from thinking of individuals as unique or distinct. There is One. Within that One you have an unfathomable number of beings, each with their own talents and abilities. The One is "All powerful" but doesn't act so based on how you might think a Jehovah style God would act, being all powerful.
5. Is there such a thing as evil? Are there evil deities?
Which side of the magnet is evil, north or south? They are one in the same and without one you cannot have the other. If you cut the south pole off, it just starts right where you cut.
Again we think of Evil as these things that happen to us or others that are harmful, but through harm one grows. If you never burned yourself on the stove, would you really know not to touch it? Maybe you didn't burn yourself but got close enough to feel it's heat, so you didn't go all the way to the one polarity, but got close enough to conceptualize what might happen if you got closer.
There are men all around the world that would wish evil upon another, but IMHO, the view is more of a dance between two polarities, each necessary for the experience, and each necessary for the other to exist.
"No rain, no rainbows" ~ Kimo (Hawaiian rules to live by)