Let's deal with the unreliable/untrustworthy narrator. (My various dictionaries are happy to accept these two words as synonyms.) AP is deliberately opaque at times in order to reveal the story in her preferred way. In particular, Jack's story is held back, hinted at rather than described in any detail, therefore it is to be expected that there is no backup evidence to support the remark that Jack "had been riding more than bulls, not rolling his own". There is also no backup evidence which clarifies what he's later spending his money on when away on those business trips. We could argue that he's lying when he admits to going to Mexico, and even that the mention of the ranch neighbour is all fantasy, since we never actually see or unequivocally get told about what's going on.
The issue is, I think, not whether backup evidence is
expected but whether it is
provided. Using the writer’s methods to support the claim that backup evidence is
not expected seems to be a deflection from that point.
The interjection is the only time in the story that the narrator directly contradicts something Jack has
said. #
In two of the cases you mention the narrator doesn’t insinuate that Jack’s lying when he tells Ennis about going to Mexico, or that he lied when he told his father he intended to bring “a ranch neighbor a his from down in Texas” to “help run the ranch.” Both men accept, based only on Jack’s word, and without any external supporting evidence being provided, that he’s telling the truth, as does the reader and, presumably, the narrator, who is notably silent in each case.
As far as the third example is concerned it is the
narrator who mentions Jack’s buying trips (and who also provides some evidence regarding how he spends his money), and
not Jack. He might indeed be spending his money on other things—but unlike the case of the motel interjection—the issue of
expecting further backup evidence doesn’t arise. In this case the reader accepts that his dental work, rather than a detailed itemisation of the expenses he incurred, is sufficient, and certainly so as Jack himself never mentions how he spends his money, or that he even
has money to spend.
Readers with a tendency to be suspicious of anything Jack says and does, even without the narrator’s “hints,” might believe—simply to find evidence of his untrustworthiness—that interpreting his words and actions is a fruitful way of passing the time. But doing so simply because they believe that the writer’s style permits them to forever look for increasingly more microscopically-hidden clues about his behaviour, even where none actually exist, brings everything we’re told into question.
Jack provides his own “backup evidence” that he doesn’t “do it with other guys” by providing a valid explanation about his belief in the importance of the mountain in his and Ennis’s lives. Ennis’s question comes out of the blue but Jack’s repudiation of the suggestion, and his
immediate provision of an authentic supporting argument—without even a pause for thought—demonstrates his straightforward truthfulness. But in this case his word is not enough for the narrator, who intentionally casts a slur on his honesty. The obscurity of its references is sufficient in itself to raise doubts about its veracity in the mind of an alert reader. The vaguely sexual implications alone bring into question what we’ve come to know about “who-does-what-to-whom” when Ennis and Jack engage in sexual activity.
That “it’s to be expected” that
no supporting evidence is provided for Jack’s
alleged misbehaviour is an extraordinary claim, Marian, especially as it’s the only time that Jack’s honesty is so directly impugned. ##
#
If that's incorrect, I apologise. ##
Ditto.