Lou, the gospel of Luke has this intro:
1 Inasmuch as many have undertaken to compile an account of the things [a]accomplished among us, 2 just as they were handed down to us by those who from the beginning [b]were eyewitnesses and [c]servants of the [d]word, 3 it seemed fitting for me as well, having [e]investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus; 4 so that you may know the exact truth about the things you have been [f]taught.
There are people much more expert in these things than I, but I will state my understanding: These gospels were all written down within about a generation of Jesus’ death, when people who had seen everything were still alive. We don’t today say that translations from english to french are invalid or even inaccurate in general on cereal boxes or Canadian legislation, so saying that there were translations resulting in filters would be invalid.
Also, the gospels do not differ among themselves as much as you imply. Rather there is difference in emphasis of what is reported. The purported differences often don’t exist when examined closely, and in the very few cases where they do exist, are relatively insignificant. There is also no evidence that the views of the gospel writer has distorted the narrative, although it is true that each gospel book has focussed on different aspects of the gospel message.
Your generalizations about the OT are impossible to answer specifically, as most generalizations are. But I would in turn generalize to say that the OT is not a transmission of legends, but of actual events. In addition, the events, people, etc. are not myths, but are true. So there you have one generalization countering another, I’m not sure to what purpose. But the nice thing about generalizations is that it avoids the examination of actual evidence, data, and arguments.
As far as the atrocities of the Old Testament, yes, it is difficult to understand. However, it is interesting that one of the prevalent pagan practices of the time was to offer babies as burnt offerings to their god Baal or Dagan. In addition to any other atrocities that were done by people at the time, it is not illogical to think that these people deserved to die for their atrocities. God also threatened his own people that if they did the same thing, they too would be destroyed. And in fact, eventually, after they had worshipped these pagan gods for many years, God finally destroyed Israel as well.
On the other hand, if you read carefully in the old testament, you will find that Israel was told to treat these foreign girls well, not to abuse them, and to take them as wives (Deut 21:10-14), and not sell her or treat her like a slave.
To treat the alien and fatherless with justice (Deuternomy 24:17-22) To stone the rapist (Deut 22:25-27) Although there are some “questionable” (to us) solutions, they are mostly due to the difficulty of deciding between various accounts of truth, like Solomon having to deal with two women who both claimed a baby as their son.