 
 
The argument goes as follows: Q-oid quotes appear in John, but not in
the almost codified way they were in Matthew or Luke. However, they are
considered to be similar enough to point to knowledge of Q as such, and
not an entirely different source.
 
 
 
We are talking date of texts here, not the age of the authors. The usual
explanation for the time order of Mark, Matthew and Luke does not consider
their respective ages. It says Matthew has read the text of Mark, and Luke
that of Matthew (and probably that of Mark).
 
As it is assumed that John knew the content of Luke's text. The evidence
for that is not overwhelming, admittedly.
 
 
 
When they are from about 200, why do they shed doubt on the order on
putting John after the rest of the three?
 
 
 
Sure, an original together with Id card of sender and receiver would be
fine. So what's that supposed to say? Am I missing something?
 
 
 
That John was a disciple is not generally accepted. The style and language
together with the theology are usually used as counterargument.
 
The argument that John was a disciple relies on the claim in the gospel
of John itself. Is there any other evidence for it?
 
One step and one generation removed is bad even in our times. Compare that
to reports of similar events in our century in almost illiterate societies.
Not even to speak off that believers are not necessarily the best sources.
 
 
In other words, one does not know what the original of Mark did look like
and arguments based on Mark are pretty weak.