Your claims are all very well, but you have no real proof that anything has been added or subtracted to the bible in such a way as to change its core message. Why would you be puzzled to find verses referring to One God and also the Trinity in the same book? Is it because as you understand it the trinity represents three gods? This is your error of comprehension, maybe because the Quran has given you a rather muddled view of the trinity and implies it means three separate gods.. (If this is an incorrect assumption on my part I ask your forgiveness) yet Christians never did believe in three gods.. There has only ever been ONE God. So there is really no contradiction as you believe there to be.

You are of course free to interpret things how you wish, although I'm not sure why you feel the onus is on yourself to prove false the sacred scriptures of others. We don't ask you to believe any of it.. :)

Peace.