Trevor, you have brought up some very interesting points. I agree wholeheartedly that the quote on the Trinity that you included in your post is gobbledegook to most people. I have a Ph D. in Biblical Studies and can hardly navigate through what was being said. It's so complicated I couldn't say if I agree with it or not.
Perhaps such things make you understand why many Christian groups, including the one with which I worship, do not have creeds or doctrinal statements. In keeping with that spirit, I'm going to try to address some of your concerns just using the Bible.
The first issue is the fact that God insisted throughout the Old Testament that He was the only God. Monotheism is the most basic tenet of both Judaism and Christianity. Period. This concept excludes the idea of persons or personalities with different origins and beginning points in time, such as Mormonism teaches. So the foundational truth of all of the Old Testament, at least, is that there is only one God. All of Christian theology rests on that basis.
While there is only one God, Scripture does, as you pointed out, show us that there are personalities, who are divine, that see themselves as having some differentiation. Not gods, but Persons in God. (Or maybe Persons of God?) The big difference between LDS doctrine and Christian doctrine is this: Jesus is not a God. He exemplifies how in the Godhead someone can be divine and not “a God.” He is and always has been divine. He didn't attain godhood nor was He created and exalted by some other Being. He was God from the beginning of all things (John 1:1.) Remember, please, the diagram on the other post.
It's true that the word Trinity does not appear in the Bible; nor does the phrase “three in one” or other things we've invented to try to linguistically capture the concept of a Being who has told us from the start that we cannot understand Him. His judgments, He tells us, are unsearchable, and His paths beyond any tracing out; His thoughts are not ours, and His ways are not ours. We can only access Him through the glimpses He allows.
The triadic nature of God is echoed throughout His creation. Reality itself is tri-partite: the part you see, the part you don't see (the eternal and invisible) and of course the necessary link between the two that allows us to know Him. Our thinking processes are triadic — facts we access by linking them to our representations (such as visual icons and words both written and spoken.) If this area of inquiry (known in the secular world as semeiotics) is of any interest to you I can send you a more complete explanation of this point, but don't want to “lose” you over it. Look here for the “basics” lessons on Representational Resources.com (start with lesson #4 of Introduction to Representational Thinking) if you'd like to explore this idea of the triadic nature of what God is and has created.
Even human beings are tri-partite while being singular. There is only one of me, but I have three parts. I have a body. If somebody steps on my foot, “I” hurt. I also have a spirit, the thinking part of me. I also have a soul, something that can be lost through disobedience. (Remember, “what does it profit a man to gain the whole world and yet lose his soul.”)
You asked who Jesus was talking to when He prayed. I have tried (and not done a good job I fear) to convey that He is multifaceted and incomprehensible. But God can have inner dialogue just like we do. A very good example of such inner dialogue is Romans 7. There Paul's fleshly nature is warring with his spirit. It's not his body — the body can't make you do anything. But he has an inner war going on. What is arguing with what?
Now, that's not a precise analogy because there are three personalities all of whom can be properly called “God.” One God. But they can all be God in the way that my body is me, my thinking processes or spirit are me, and my soul is me.
Trevor, I hope this helps.
