What were Jesus and early Christians like?

[Mudhammam]

Wrong. We know that gMark was using at least one original source, now lost. Then there is the Q material used by both gLuke and gMatt, at least some of which is demonstrably from a common text, also now lost. Then there is the material exclusively in gMatt and gLuke, some of which may be what you call "fanfic" (actual textual critics call it midrash) and some of it could be from further lost sources. Then there is a whole slew of stuff in gJohn that is not in any of the synoptics, which must have come from somewhere.

My brother has spent quite a bit of time reading early church material and researching different hypotheses regarding the original sources for the gospels. He suggested to me the idea that all four gospels might have been working off an earlier narrative.

But unless there is something substantial discovered, it seems to me like the speculations about when tales of Jesus originated in either oral or written form can go on and on, and it makes little sense if we assume that Jesus really was crucified sometime around 30-33 C.E., that immediately these elaborate stories began circulating...or perhaps that was a common feature of recently deceased Jewish wisdom teachers whom some small sect revered?

The idea that the other gospels are just elaborations on or embroiderings on gMark and it can all be traced back to gMark is simply wrong.

Why could not Matthew and Luke had access to Mark and Q and wrote their narratives primarily based off those, with their own agendas and anecdotes marking the rest of the differences between them?

This is most likely. The fact remains that both gMatt and gLuke are using this "fanfic" stuff to "explain" how a Galilean from Nazareth could be the Messiah and born in Bethlehem in Judea. So the question is - why? Why are they going to all this trouble to "explain" how he could be the Messiah and still be from a nowhere town in Galilee.

To unite different factions of Jewish and Gentile Christians who had different expectations about the Messiah's birthplace and because Mark's Gospel was already considered authoritative to a substantial audience?