Could Chapters 15,16,17 of the Fourth Gospel be interpolated?

I have for some years been fascinated by The Fourth Gospel (called The Gospel of John) - and re-reading it again I was struck by the sense that three chapters in between Jesus's last meal with his disciples and leaving to the garden of Gesthemene, had the quality of an interpolation. 

The first clue is that Chapter 14 ends with "Arise, let us go hence"; and Chapter 18 begins with "When Jesus had spoken these words, he went forth with his disciples over the brook Cedron, where was a garden, into the which he entered, and his disciples." On the face of it, it seems as if 14 should lead straight onto 18. 

I also notice that substantively Chapters 15 and 16 do not add anything core to the teaching; but function more like a commentary - an elaboration and re-explanation on what went before. 

And similarly Chapter 17 (which takes the form of an extended prayer) mostly has Jesus asking for what he has (apparently) already promised.

So I would mildly conclude that Chapters 15-17 are likely not based on writings by Lazarus: author of the now lost but original and contemporary eye-witness account that was "Fourth Gospel". 

Does this kind of "analysis" matter? Well, it might, for some people, at some points. 

The most important aspect of reading the Fourth Gospel (or Scripture, more generally) is that we get from it the most important, vital, knowledge of Jesus and the main "things" he did and taught. 

By my understanding this is only very seldom achieved because people fail to focus on IV Gospel and get their core guidance from other parts of the Bible - this means that "Christianity" has-been and is serious misinterpreted. 

Secondly, any attentive, active, positive personal engagement with Scripture is almost bound to become something of a living (and, we hope, direct) interaction - including discernments and evaluations of many kinds. 

But, exactly because it is alive and present, what is significantly valuable for me/ here/ now might well (and unavoidably) be a matter of indifference - or negative - for other people, in other circumstances...

Such is the nature of all communication, because of its intrinsically indirect and symbolic nature. 

Note: Of course, as usual with the Fourth Gospel, there are beautiful and lumious sentences even among those parts that are apparently later additions, and some inauthentic parts perhaps helpfully emphasize the core teaching such as Chapters 15-17. And there are lovely, poetic, aspects to the words; even when sections contradict the essence of the message!