After reading Unfinished Tales, I think you are even more correct. From the Essay on the Palantir, appendix item 21:
Looking one at the other the would change "thought" - not their full
or true thought, of their intentions, but "silent speech", the
thoughts they wished to transmit (already formalized in linguistic
form in their minds or actually spoken aloud), which would be received
by their respondents and of course immediately transformed into
"speech", and only reportable as such.
This starts to make sense. As you say Denethor was pretty powerful, and his rightful ownership to the palantiri made his use of the palantiri more 'uncorruptable'. The exact passage says,
Denethor remained steadfast in his rejection Sauron, but was made to
believe that his victory was inevitable, and so fell into despair. The
reasons for this difference were doubt that in the first place
Denethor was a man of great strength of will, and maintained the
integrity of his personality until the final blow of the (apparently)
mortal wound of his only surviving son. He was proud, but this was by
no means merely personal: he loved Gondor and its people, and deemed
himself appointed by destiny to lead them in this desperate time. And
in the second place the Anor-stone was his by right, and nothing but
expediency was against his use of it in his grave anxieties.
Plus, if Denethor was corruptable and could have his mind read, then certainly Pippin would have had his mind read too when he touched the palantir. We know that this didn't happen and Sauron had no idea of the true mission of the Fellowship and thought Pippin was the ring bearer.