They are copies of copies, and copies were known to be altered for religious/political purposes.
Thankfully, though, textual criticism is a well-established discipline in the field of historical studies. More importantly, "...the textual critic of the New Testament is embarrassed by the wealth of material. Furthermore, the work of many ancient authors has been preserved only in manuscripts that date from the Middle Ages (sometiimes the late Middle Ages), far removed from the time at which they lived and wrote. On the contrary, the time betweeen the composition of the books of the New Testament and the earliest extant copies is relatively brief." from Metzger and Ehrman, p. 51.
What does this mean? It means that for the most part we have a very, very, very good idea what the original NT texts looked like. For the cases where there is some doubt, or we really aren't sure, any good criticial greek edition will let the reader know. Galatians 1.19 isn't one of these cases. On textual criticism in general and NT textual criticism in particular, see
Metzger, Bruce M., and Bart D. Ehrman. The Text of the New Testament: Its Transmission, Corruption, and Restoration. Oxford: Oxford University Press, 2005.
Ehrman, Bart D. Studies in the Textual Criticism of the New Testament.
Leiden: Brill Academic Publishers, 2006
No, they aren't. For one thing, we don't need the gospels at all to show that Jesus is historical. We just need them to find out anything beyond that. But the real issue with the above is the word "assumption." Perhaps, given how little you have read, you may have gotten that impression, but it isn't accurate at all. What you call an "assumption" has been a debated issue in scholarship for well over a century. Yet those experts who are extremely skeptical of the reliability of the gospels (e.g. Bultmann) nonetheless believed we could do more than just establish historicity of Jesus. Why? Because for one they know enough about ancient genres to not propose idiot ideas about Mark creating a new genre that everyone misunderstood, like R. G. Price did. More importantly, it was realized that even if one takes the Bultmannian view that the early christians freely attributed sayings and teachings to Jesus and had no interest in any historical figure (a view long since realized to be untenable), it is clear that, unlike with the various cults of the ancient world, a single founder stood behind the movement, and all the literature looks back to one person.The problem with Jesus is historical arguments is that they are based on the assumption that the gospels are reliable histories
The historical value of the gospels isn't assumed, any more than it is with any other text from the ancient world. In fact, they tend to be judged more critically. This is why we compare the gospels to other genres and find they match up with other historical works far better than with literary expressions of myth. This is why we look at the sociology of religion to see how movements, sects, cults, etc grow and are structured, and find that independtly of Jesus himself, the nature of the movement needs a single founder which, conveniently enough, all the texts give us.
Of course, it is easier to write off two centuries of scholarship if you say it is all built off of an incorrect assumption. So what if the statement is completely inaccurate?
Even if Paul stated "brother of Jesus" scholars would no doubt question the validity of that line because of everything else Paul and the other epistle writers state that contradicts such a notion.
I've already addressed this, and so has scholarship within the field of historical Jesus research. Only the mythic questers, who find that the best approach is to assume Jesus is a myth and then massage and distort the evidence to get there, have a problem with Paul's statement (and as Price notes, this is because it is devastating to the mythic hypothesis).
Mythers don't dismiss scholars because they supposedly have a Christian bias, mythers don't assume that the gospels are historical accounts of actual events, that's the difference.
"Mythers" assume the gospels aren't. Scholars have examined the issue from all sorts of angles for the past two centuries. They don't "assume" anything.