I should say that I'm not a Christian. (I might become one the future, but for now I'm definitely not.) So I have nothing to gain by proving or disproving anything.
Well, the near-silence on the life of Pilate outside of the NT shows that most historical records simply disappear over time. Until the Pilate Stone was discovered in 1961, researchers were doubtful of Pilate's historicity. I know this because I've read books published before 1961 and the general consensus pre-1961 was that there was zero reliable evidence of his existence. Even today, a few doubt that the Pilate Stone actually says his name and that he existed.
Modern scholarship? Well, modern scholarship believes that Jesus was an historical person, and for good reasons. Modern scholarship denies your entire premise that an historical Jesus is a myth. Any "evidence" you provide for your premise is automatically suspect; if all serious researchers reject such evidence, then why should I not reject it? I admit that I haven't done serious reading into the "Jesus Myth" hypothesis, but that's only because all the books on Amazon seem to be written by dubious people with an agenda. (I won't read "science" books by Evangelicals and Muslims who deny evolution for the same reason.)
It seems that you're cherry-picking reason to accept or reject "modern scholarship" when it suits your agenda. When it comes to evidence of a census, you say, "modern scholarship this and that," but when it comes to Jesus' historicity, you turn around and say, "Modern scholars are wrong." How is that any different than creationists who cherry-pick the science that they like and reject what they don't like? It's funny how, when I google the phrase "Jesus Myth," a good number of hits come from atheist websites. That's a bit telling, at least regarding the motives of this movement. (In other words, how is such a conspiracy theory so popular among those self-proclaimed paragons of reason?)
Regarding the census in the gospel of Luke, this subject has always been of interest to me. It's true that there is legitimate reason to think that the census never occurred, but I wouldn't call it a "modern scholarship consensus." For a long time, I thought that it was simply a fabricated event, but when I did a bit of book reading through material from those who claim to provide evidence of a census (and there is a significant minority of scholars who now make this claim), the evidence for an historical census is [somewhat] compelling. I won't go into it here, but one website lays out a very brief survey of some of the evidence.
http://www.askelm.com/star/star014.htm (It's a religious site, but the points in this particular article are legitimate.)
I've read all of Justin's writings. It's been a while, so my memory is a bit rusty, but I can assure you that Justin was not some wacky loon making up stuff. In Trypho, we see that, prior to his conversion to Christianity, he was a well-educated philosopher. According to his Martyrdom, we know he lived in Rome and therefore possibly had access to the records he mentions. I don't think it's correct to assume that Justin "lied" about or "naively believed" in the existence of an Acts of Pilate and a record of the census. He wasn't a moron who converted to Christianity "willy nillly," and his Apology was an intelligent defense of his newfound faith. Since he (and Tertullian) seemed absolutely confident such records existed, I have no reason to think that he didn't personally confirm their existence. Sure, it's possible that what he saw was pseudepigraphical or whatever, but I don't think he just "assumed" it was there.
That's a pretty loaded question.
Can you give any evidence of this having happened? Where did these conniving Christians "alter ancient documents"? Can you give some examples of this happening? The only example I can think of is Josephus' mention of Jesus, and even the actual extent of that is debatable. But I certainly don't equate pious fiction written centuries after Christ as being a willingness to "provide a real record" of Christ.
Maybe the end of the Gospel of Matthew, where Jesus mentions baptism in a Trinitarian formula? Eusebius leaves this part out when he quotes the passage, but what some forget to mention is that, during Nicaea, he was of the party representing semi-Arianism. And more importantly, there are earlier Christian sources quoting that passage of Matthew with the Trinitarian formula intact.
Perhaps you're making the mistake of lumping early Christian texts with those of later centuries. It's true that, in the 2nd-5th centuries, many extravagant texts were written, but the texts we have in the NT were regarded as authoritative for a couple of reasons: they were known universally, and they were known since ancient times. People back then were not "stupider" than us "moderns." Critical thought didn't magically come into being in the 19th century.
A certain school of modern scholars do what the atheists today do when categorizing Christians. They essentially put them all on equal footing and make them out to be equal claimants to the Christian name. They say that the Catholic Church is just "another denomination," like the Baptists or whomever, but the Catholic Church clearly has historical precedence over Protestants.
Regarding the Gnostics, they were disregarded by ancient Christians for a few reasons. One reason is that they existed before Christianity and merely latched onto the rapidly-growing religion. Another is that they taught things that contradicted the Christian scriptures (that there were two gods, that salvation required gnosis, that the god of Jesus was "out of touch" with reality, etc). A third reason (and the most important reason) is that, while the Christian faith existed uniformly and universally (yes, it did, contrary to the school that claims otherwise), the Gnostics existed in isolated groups whose beliefs contradicted one another, all of which were novelties anyway.
Irenaeus of Lyons wrote a famous book Against Heresies (circa 180) in which he argues against the Gnostics directly. In Book 1, Irenaeus pretty much demolishes various gnostic groups and their various beliefs by showing their origins and absurdity. In Chapter 10 of Book 1, he clearly makes a point that the above-mentioned school like to ignore, which is that
According to Irenaeus, in a book addressed particularly to heretics, there was a "mainstream" Christian church in ancient times and these gnostic groups were objectively inferior to it due to their lack of universality, their contradictions between one another, and the novelty of their doctrines.