‘Canonicity’: the Book or the Reader?

[davem]

We respond to it for profound psychological reasons.

Its not about 'why' we respond, but what, if anything, we respond to.

Is it, for example, the one where Turin returns to slay Ancalagon or the one where he returns to slay Morgoth?

We can't know, because Tolkien didn't discover in time, but we can know that one of them is correct & the other isn't, because, given time, Tolkien would have discovered which one was 'correct'. If an explorer seeking the source of a particular river has two alternative possibilities but dies before he can determine which is correct, that wouldn't mean each of them was 'equally correct', or that people were free to choose which of the alternatives they preferred. And that would still be the case even if no-one else was able to go find out which is the case. We have to accept that some facts about Middle earth will remain unknowable to us, but that in itself doesn't mean the place itself has no 'objective existence' or that the response the texts evoke in us isn't to something 'real'.

Perhaps the problem is that you're arguing for the sole 'reality' or the texts (if I understand you right), while I'm arguing for the reality of what the texts refer to. So for me, 'contradictions' in the texts are not relevant.