First, having different writing styles means nothing.
If one is writing to a different audience, or dealing with a different subject, it is understandable that the authors style would change.
Two, regarding David's conquest,
Excavations have revealed plenty of traces of the acquistions and expansion of the kingdom of David. There is a clear trail of evidence which accompanies his advance, including the burning evidence of the cities of the Plain of Jezreel....Archaeologists from the University of Pennsylvania have dug up on these sites of ruthless fighting, shattered temples, deep layers of ash on top of ruined walls, ritual objects and pottery belonging to the Philistines. David's vengenance administered a crushing blow to the city which had compassed the shameful end of the first king of Israel, a blow from which it did not recover for many years to come. (The Bible as History, Warner Keller, pg. 196)
Regarding the superscriptions on the Gospels,
The dating of the emergence of a four-gospel collection has often taken its bearings from the particular form of the superscriptions or titles of the Gospels: 'The Gospel according to X'. Martin Hengel argued from this that the titles must have originated, quite early, from the practial necssity of distinquishing among such documents when multiple gospels were at hand in the archives or litugical libraries of particular communities. Hence the titles were not late additions, but belonged to the gospels in their earliest period of circulation and had historical tradition behind them....He believes that the gospels originally circulated individually yet rapidly, so that by the early second century(emphasis added) they were present together in church librries of major centers;...(The Canon Debate, Lee Martin McDonald, James A. Sanders, editors, 2002, pg.278)
I suppose for some such as yourself, however, ignorance is bliss and that is why you choose to remain so.
The first attempt to create Chrsitian 'canon' was made by none other than Marcion (the heretic some Baptists like to call their own) in the 2nd century, so your sources are 'beliefs' as you say (probably without relaizing it):
There was no Christian canon in those days. That's some woodoo scholarship you are citing. The fact is that even Athansius whom you love so much believed at least two apocryphal books to be scripture. But that was two hundred years after the fanatasy you quoted about the early 2nd century. At that time, all sorts of books were being read as scripture in early churches.
One Clement (96 AD) paraphrases the book of Hebrews but does not cosnider it scriptutre. the only thing he considers scripture is the Old Testament. That's because the Gospels were still being written.