Did Denethor see Frodo captured in the Palantir?

[Zigûr]

Originally Posted by Alfirin

On the other hand, if he could play into Denthors fears so much as to focus on eliminating Aragorn as soon as he arrived or even go so far as to subtly insinuate himself deep enough in Dethethor's mind to, in a virtual sense, suggest the following "You know, if you turn traitor and ally Minas Tirith WITH me, we can get rid of this last threat to your rule and you and your sons can rule the city forever unthreatened. My servant made your son sick, do you not believe I could make him well again. Continue to oppose me and you are doomed, one way or another. Join me and your line is secure.

The idea that Sauron showed Denethor his doom in Aragorn's rightful kingship is an intriguing one, and I quite like your thinking. This specific bit above about Denethor actually turning traitor, however, I consider to be most unlikely. I'm sure we're familiar with Letter 183 regarding Denethor:

It had become for him a prime motive to preserve the polity of Gondor, as it was, against another potentate, who had made himself stronger and was to be feared and opposed for that reason rather than because he was ruthless and wicked.

Hence why I think Denethor was in a no-win situation: submit to Aragorn and lose his position, or submit to Sauron and lose it. I think he hated Sauron too much as his chief "political" rival to ever consider alliance with him or even feign it the way Saruman did. Sauron and Aragorn would probably be no different in his mind: two powerful people who could oust him from his position as (in his prideful view, at least) the eminent leader in Middle-earth.