This post contains lots of interesting stuff but it only relates to one aspect of what were discussing last Monday and I am going to try to explain how i think it might connect. Thanks to Anna for starting the discussion off so well!
Mr Pip is narrated retrospectively by a young woman who is telling about her experiences between the ages of 13 and 15 (‘skinny thirteen year-old’ on the first page). In fiction retrospective narration can work in many ways - sometimes a lot is retrospective assessment set against the past/present experience of the story and therefore moving between different understandings of a situation, sometimes the focus is more on the innocence at the time and although in the past tense reminds us very infrequently of the longer perspective. This latter manner is how Mr Pip is structured - so as you say Greg it is a useful voice in which to tell the events because it can engage readers who are also ignorant of the history and context of this island and because the young perspective is vivid and describes in a detail (the beach, Mr Watts, the stories, etc, etc) that is useful to evoke that world for those unfamiliar with it. Often child perspectives are to bring alive what we no longer see, or judge too much see. (And I agree with what you say about the choice of a girl to tell the story - keeping the narrative out in the open and so on).
However both 'convenient and versatile' imply something else, you seem to be saying that this was a good way for a writer with himself a limited knowledge to tell a story – perhaps, but isn't that worrying: aren't gaps and silences left intentionally in a story to hint at knowledge and understanding that surpasses what is actually included.
That makes me come back to on the one hand the tension between the retrospective narrator and the protagonist - remember Jenny commented on how in Great Expectations Pip comments in complex and often self-critical way on what he is telling. This mix can thicken a fiction - make it more meaningful as we think about the difference between different perspectives.
Another issue is the one raised about Sargeson and the distance between Sargeson's narrators (and often his characters) and the perspective of the implied author. Sargeson's work hinges on this distance - he uses an unreliable narrator ( I don't think Matlida is quite that though we th reader do think beyond her naivete at times) - and the narrator is telling a story (boxes within boxes) of someone he has met or known. It's usually a story which he the narrator is moved by but dosn't quite know why - ie he emotionally feels its significance but can't explain why or what it is. The difference from what greg is suggesting is in Sargeson’s case that this is an extremely conscious strategy for telling a story which is particularly designed to comment on what Sargeson the author thought was a judgmental and impoverished society in which those real experiences of joy, love, revelation, purposefulness were experienced by ordinary working people ( particaularly men)denied and destroyed by a utilitarian and judgmental (puritanical) middle class. Hence he is dignifying a real experience of the world by creating these inarticaulte narrators. He wrote mostly short fiction often - a good genre in which to sustain this tension. You acn of course deconstruct this also -s howing that the desire to show without telling, to hide knowledge and to write plainly is in fact a continuation of the puritanism he was trying to break away from.
So the question comes back to MR Pip - does Lloyd Jones use the naivete of Matilda's to some purpose - other than a useful way to tell a fable or fairystory. Is there a tension between Matilda's new understanding of the world and her first one. I think to give him his due at the end Lloyd Jones is trying to show that story can come via a pathetic example – that is it arises from NZ in the form of a rather lost and undirected actor in an arid community who gets involved with the lonely island girl living next door - Mr Watts.
But somehow I am not convinced both because the narrator's rethink has been delayed without building up a questioning tension as Dickens or Sargeson do in different ways and because the pattern of story- telling in the main part of the book has been so much the pattern of colonial appropiation, misrecognising or leaving out and erasing the culture of the island, and privileging the power of story as it emenates from the Western classic - even if its conduit, Mr Watts, is unlikley.
I hope I have explained this - this last part probably needs better articulation!
The choice to go back to the island is, as Bruce said, also interesting - why? And why escape alone - in the initial plan?