Before the Quran, Allah were sent prophets to send messages of Allah in the form of Holy books (book of tawraat, injeel, ...etc) but people of that time changed these books (torah, bible...), why did they do that? Why did they change what Allah/prophets sent to them? Why did they change the true name Allah to God?
Because people are people, they are flawed, and much of it was an oral history before it was written down. Indeed a lot of the Torah, for example, is also things like poetry, history, poetic views on creation which are not accurate, and so on.
There were many events and people involved over hundreds of years with the creation of the Torah. The overall message though is accurate and stands out, such as the oneness of God (Deuteronomy 6:4, et al). Allah expected these things to happen, which is why Isa (SAW) said according to John:
But the Counselor, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you. - John 14:25-26
So Isa (SAW) tells that there will be another messenger/prophet after him. There are other verses which mention it as well, but that's beyond the point of my answer.
Overall, if you look at what is in the Dead Sea Scrolls and Septuagint it's not too different than what you find in the Torah and Bibles today, but due to translations a lot of things have been wildly misunderstood and misinterpreted.
This is why it's so important that tafsir (interpretation) is done in the original classical Arabic. We can already see how translations have caused issues with people not understanding what is actually in the Qur'an. This is an important tradition that must be held on to, because understanding the original language not only allows one to see allegory and double meanings which are absent in translations, but also it binds Muslims together.
That does indeed bring me to your last question, why is Allah called God. That's because it's just a translation of his name. Allah is Arabic contraction of Al Ilah (The God, The Lord). In prayer it matters, but in discussion, especially in the west, I think it can be good to say God, because it helps other People of the Book who are not Arabic speakers understand we are all talking about the same God, Allah, 'El (Hebrew).