The first answer to your questions is that RastafarI does not have a defined doctrine of theological beliefs that are subscribed to across the board. Some Rasta may say yes to all four questions, and others may say no to all four. Each person has their own mental capacity and needs to use their own mind to reason on these matters. Paying attention to the reasoning of each individual is going to get more accurate answers than applying one person's answers to the entire movement.
The next answer is that beLIEve is not usually a part of RastafarI reasoning. Better to Know, I&I say. So when you phrase those questions as 'do you believe' such and such, some people are going to say No, because it is not about belief.
I will answer these four questions with my personal views, now that I have given the disclaimer that this is a reasoning and not defining all of the ideas of all RastafarI.
1. Yes that is the beginning of the use of JAH in Rastafari, because in older versions of the bible, the name JAH is used ("Extol him by his name, JAH.") However, when I say JAH I am not referring to a bible god who loves the smell of burning flesh, orders his people to murder babies and womban in genocide of other tribes, and sets up a metaphysical payment system in which only death and blood can redeem his beautiful creation from sin. I am referring to the Almighty of the Universe, the seed of divinity that is in each one of us, and I am referring to His Imperial Majesty Qedamawi Haile Selassie.
2. I am very interested that your reasoning for "his Son" I am guessing this refers to Jesus? coming to earth, not to be sacrificed for mankind, but to teach them more about the writings. From reading the things attributed to that character, if he was a real person in history, I would agree that his mission (judging by his actions) had a lot more to do with teaching people about their divine link to the Creator than it had to do with spilling his blood for them.
3. It is clear in those writings what the jesus character is trying to tell people. "I am in the father, and he is in me, and I am in you." Yes the Almighty is One with all JAH children, not only the man in that story; even in the story this idea is apparent.
4. Yes absolutely, the message has been perverted, this is a fact. Nicaea 325AD is an early and clear example of mainstream religion inventing spiritual dogma and filtering writings to go along with their message. This is where the "born of a virgin, the only Son of God, the father, son, and holy spirit," doctrine comes from.
I am not offended by your questions, just as I hope you are not offended by my answers.