I know you don't like to consider materials from outside the text overmuch, but I'd quote Letter 183 again: "He had become a 'political' leader: sc. Gondor against the rest." Whether he
is a politician, Tolkien says Denethor was that kind of leader. The context is that Tolkien says Denethor resents Aragorn, not because he's unfit, but for the opposite reason--that he's so fit as to displace him. Denethor resents anyone who is greater than he, which Gandalf certainly is.
As for the notion that the scroll is written in some obscure language that Denethor does not know. This is a long stretch. First, we have no evidence that the scroll is written in an obscure language. We have no knowledge of what languages Denethor speaks. Even if we make the stretchy assumption that Gandalf knows that the scroll is in a language that he knows Denethor does not speak, this proves nothing. Denethor, as Steward of Gondor, undoubtedly has translators available who could parse any language that Isildur could write.
This is not a stretch at all. In fact, the text is rather explicit on the point.
"And yet there lie in his hoards many records that few now can read, even of the lore-masters, for their scripts and tongues have become dark to later men. And... there lies in Minas Tirith still, unread, I guess, by any save Saruman and myself since the kings failed, a scroll that Isildur made himself."
The implication, here, is clearly that the scroll is in a tongue that has become "dark to later men". So dark, perhaps, that no one could translate it for Denethor, or had ever done so if they could.
Why does Gandalf even tell this story? It seems a very undiplomatic story to tell. Also un-necessary in any scenario I can think of. Gandalf is going out of his way to cast shade on Denethor. Why?
Because it's important to establish whether anyone else has this knowledge. He suspects Saruman is the only other person who does. Saruman's ring-lore is of present concern, and will grow in importance. Tolkien uses this moment to suggest Denethor as an antagonist, and one who doesn't much like or listen to Gandalf. Gandalf further establishes the limits of the line of Stewards' lore. Maybe that's not terribly polite, but it's important. They have dinged Boromir on that point several times already. You have to be honest about your allies, and to them.
A reading wherein Gandalf is either wrong or dishonest makes less sense, and I can't say what Tolkien would seek to accomplish with it. That's part of the reason I don't think it's correct.