Not really. Denethor was the trueborn steward, whose ancestor had been officially appointed by the King, and though it isn't mentioned in the Lord of the Rings, the essay on the Palantiri in Unfinished Tales says that stewards were often deputized to use the Palantiri. So Denethor had the right to use the Palantir of Minas Tirith. But he didn't have the wisdom to realize that Sauron was manipulating what he saw.
I agree that the movie portrayal was totally unlike the Denethor in the books.
> He knows Sauron is trying to manipulate him
To some extent, yes. But I'm not sure he fully realizes what's going on. For example, he sees the fleet with black sails coming up Anduin--but he didn't see any of the events that led to that fleet being taken over by Aragorn and his followers? He could have.
> because his claim is weaker than Aragorn's
I don't think this is given as a cause of Denethor's doom in the books.
> because he keeps using it repeatedly over the decades
This is mentioned in the books, yes.
I believe there is no shortage of aspirants.
Of course there isn't one; the notion of the "rightful king" in Middle-Earth does not have a real world counterpart.
Tolkien might have believed it did, since he was a Catholic and might have believed in some version of the divine right of kings that the church supported for many centuries. But even then, the power the "rightful king" has in Middle-Earth is very limited. There is no hint that Aragorn, once he becomes King, micromanages everything in Gondor or makes rules by royal decree about everything, or even any very great number of things. The only actual official acts of his that are described are making peace with the Haradrim and the Easterlings, giving Sauron's freed servants the lands about Lake Nurnen, and pronouncing judgments of particular cases, of which Beregond's is the last. He certainly doesn't seem to be dictating what everyone in Gondor should do in their daily lives. Nor is there any hint that previous Kings did any such thing.
And even Tolkien's real world attitudes weren't necessarily monarchist. In a letter to his son, he wrote:
"The most improper job of any man, even saints (who at any rate were at least unwilling to take it on), is bossing other men. Not one in a million is fit for it, and least of all those who seek the opportunity..."
If this espouses any kind of political view, it's libertarianism.
But one every 2 million?
It does not sound too libertarian to me, but I've known monarchist libertarians, so it is a spectrum I think.
I think that was a figure of speech, the intended meaning of which was "nobody is really fit for it".
Some of these kids eventually grow up and meet people who are kind to them. They find positive lessons in real human social interactions. They leave their protective fantasy bubble behind. They eventually learn to seek justice where there was injustice.
Others never grow up. They end up seeking to fight injustice with a new form of injustice. Only this time, they get to be the tormentor.