John at least in some form could date prior to AD 70 but its lengthy discourses, realized eschatology, and high christology fits better with a date towards the end of the first century AD, and the appendix certainly recommends a date after the passing of the generation contemporaneous with Jesus (ch. 21 however is probably a later addition to the gospel).
I agree, I think that core of John was probably written before 70AD, but edited at least a couple of times before the finished Gospel, Probably because JOhn has SO MUCH material in it, it makes sense that a couple of editions would be circulating.
a feasible hypothesis is that it is pre-Christian in origin, composed as a midrash of Daniel (this would account for the many literary parallels with certain sections of 4 Ezra). Some have suggested that its original setting was the Caligula controversy of AD 40; this better fits the features of the apocalypse than the situation in AD 66-70. So it is noteworthy that the redactions that make it better fit with the situation in AD 70 are to be found in a later gospel dependent on Mark, namely Luke, and not in the Markan text as we have it.
Interesting theory, I have also read that Mark was composed at various stages and that, liek John, it was "edited" together in its somwwhat final stage.
The fact that we have two different endings does tend to suggest at least two verisions of Mark's Gospel that were circulating.
The fact that WE KNOW of these two endings and that they are dicussed and debated ( for sometime now) show, as an example, that these "conteoversies" are nothing new.