I am not Christian, however, my College was and we were required to study at least some amount of it. Also, I have just always had a personal interest, as Christianity has been a powerful force in European history, literature, and art.
From all this, what I gather on the various subjects raised are:
The Trinity
The “trinity” was developed sometime between the middle ages and the renaissance (I can’t remember specifically off the top of my head–but it was definitely after Christianity had become the cola of choice in Europe.)
It was used as a general purpose teaching tool for getting people to understand the different aspects of God. …Similarly as you could use the trinity of Life, Justice, and the American Way, as a method of teaching future generations about the core of What is Superman.
In the case of god, these are God (the big guy, creator of everything, vanquisher of evildoers), Jesus (the god who feels our pain and understands), and the Holy Ghost (the all-pervasive, general spirit of “something” that exists around us always.) These are not intended to refer to separate entities–just to serve as an overview of the basic forms He takes and all the ways in which we should think of Him and respect Him.
At the time of the development of the current Trinity, there were several other competing trinities, but this one was decided upon as being the one which best encapsulated the issue.
God’s Stance on Other Gods
God originally started as a marketting effort among the Jewish people to try and bolster self repect. At the time, the Jewish people were fully under the control of the … (B something…) … peoples, and all of their gods positioned decidedly lower in rank than their overseers. Which made sense, as the Jewish gods were along the lines of The God of the River on the East Side of Town, or the God of That Funky Gunk Between my Toes, while as the Babylonian (ha! remembered it) gods were the God who is the Sun, and the God who is the Moon, and such.
So when someone came up with or discovered God, the creator of all things (who just happened to favor Jews…), it is not hard to imagine that this was very popular among the Jewish people. “Yeah your gods may be the Sun and the Moon and everything, but our god created the Sun and the Moon, so nyah!”
The Old Testament was thus written largely in this light. Even among the Jewish people at the time, there were still several of the old gods floating around, and some belief in the Babylonian panthyon. But if the Jewish people wanted to unite and kick Babylonian tuckus, they needed to very strictly state that the Jewish people are only allowed to worship one god, and all the other ones are just tricksters, not worthy, or simply another form of Him.
So traditional Jewish practice does not negate the existence of other gods, it simply says that you aren’t allowed to concern yourself with them or else.
Later, Jesus simply reiterated the whole thing. Roman gods may exist, and they may be impressive, but God is the big guy, can’t be viewed as subbordinate to any Roman god, and you may only pay any credence to him, or else.
It was probably not until the Catholic church had such extreme power and wanted to position itself as the One and Only that the idea that only one god existed came into existance. A form of snobbery for someone who had reached the top. But previous to that, all of Christian and Jewish history believed in one all-mighty god, and the existence of more minor and irrelevant gods who were created by the big guy for ineffible reasons.
Origins of Satan
Satan does not exist in true Christianity. Or at least this existance is very suspect.
That is to say, Satan has been a wives tale and all purpose boogey man who has been so prevalent for so long, that it is now difficult to disassociate him from Christianity.
Within the Old Testament, there are one or two minor references to some sort of spiteful personage, but these are not terribly clear. They most probably exist as the parables that were inserted into the Bible were probably based on more ancient tales that had been updated and mostly stripped of older, heathen items. But only mostly.
Within the New Testament, there are some references–still mostly vague–that do accept more ideas along the line of some designated evil personage. However, these seem to be based on the same kind of thinking which now gives us the whole Satan mythos–i.e. the simple force that a thousand years of snowballing of this boogeyman character was bound to have.
Again, it was the desire of the Catholic church to hold and control power that really gave the push for the creation and widescale adoption of the idea of a Satanic character. If there was no malignant, organized force in the world, why would you need a benevolent, organized force to exist?
So essentially, Satan was an efficient ruse to continue expanding power and raising tithes.
The Satanic Mythos (or, Is He A God?)
The story of Satan is that he is a fallen angel. By this basis, he should have no power beyond what any other angel has. And as no other angel is considered a god, we thus cannot relegate Satan a god.
However, as Satan is really an outside character to Christianity and thus need not be restrained by the hard letter of the book, it really just depends on what era of the myth you tune into and who is doing the talking.
The original boogeyman character was probably a god. But as that character got changed around and he became, merely, a fallen angel, he ceased being a god. Now that the general concensus is that he was really a special entity that (while originally seeming like he was just an angel) God created with the express purpose of acting as his own nemisis and bestowed with some amount of godlike powers so as to be able to carry out that role, conceivably he could be considered as a god–if that is the way He, in his ineffible ways intended it. That is, the current status is very unclear, but definitely moving towards a godlike status in the public mind.