[Boromir88]

Ai! Ai! So tangling and messy.

I think Sauron only sent one Nazgul because that Nazgul was the closest one. Sure, he could've gathered them all together and sent them in force, but why wait? His precious, his key to absolute victory, was within his grasp! Send for it immediately!~tp

That still really doesn't make sense. Sauron may not have feared Saruman by using the ring mono-a-mono against him, but he definitely saw a weakness if somebody (especially somebody of significant power) got a hold of the Ring:

If that happened, the new possesor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done sine the making of the One Ring, and so overthrow him and usurp his place. This was the essential weakness he had introduced into his situation in his effort (largely unsuccessful) to enslave the Elves, and in his desire to establish a control over the minds and wills of his servants.~Letter 131

Unless Sauron was brainless, it doesn't make a lot of sense. I don't think Sauron would have sent all his forces to Isengard, since he had other things to deal with (Gondor, Lorien, Mirkwood, Erebor...etc) but it was definitely clear Sauron had an overwhelming mass of forces at his disposal. Had the Ring not been destroyed, just the sheer numbers and force of Sauron's armies would have crushed those who opposed him.

Sauron had many armies at his disposal...and again taking into account that the Nine could not get the job done at getting the Ring when they had the chance. If Sauron believed that Saruman had the ring (somebody who had some power), I think it would be more important to him and getting that Ring than just sending one Nazgul.

There are two possibilities I think in only sending one Nazgul...

1) As Mithadan said, perhaps Sauron still felt like he had Saruman controlled and a faithful obedient servant. (However, as Grishnakh shows, Sauron had already begun to grow distrustful of Saruman...so I don't know if this would make sense).

2) He wanted the Hobbit to get as much information from him as possible, which wouldn't require all that much to get. Considering that if Isengard was still standing, Saruman would have soon come to realize this was the wrong hobbit, there wouldn't be much need of him, and probably would willlingly give him up to Sauron. (This is also though just a bunch of possibilities and speculation).

With the questions over the palantir. The palantiri could not be made to lie by anybody. So, they were definitely accurate and reliable sources of information. What Denethor saw in the palantir wasn't lies by Sauron, they were actually taking place...But since Denethor did not have the mind, or will, to contend with Sauron, Sauron was able to control and show Denethor only what he wanted Denethor to see:

’He was too great to be subdued by the will of the Dark Power, he saw nonetheless only those things which the power permitted him to see. The knowledge which he obtained was, doubtless often of service to him; yet the vision of the great might of Mordor that was shown to him fed the despair of his heart until it overthrew his mind."~The Pyre of Denethor

In this encounter with Sauron and Pippin, I would no doubt expect that Sauron saw everything he wanted.

1) Pippin wasn't a rightful owner of the palantir, yet Denethor was of his:

These stones were an inalienable gift to Elendil and his heirs, to whom alone they belonged by right; but this does not mean that they could only be used rightfully by one of these 'heirs'. They could be used lawfully by anyone authorized by either the 'heir of Anarion' or the 'heir of Isildur', that is, a lawful King of Gondor or Arnor. [...] In Gondor latterly, as the office of Steward rose in importance and became hereditary, providing as it were a permanent 'understudy' to the King, and an immediate viceroy at need, the command and use of the Stones seems mainly to have been in the hands of the Stewards, and the traditions concerning their nature and use to have been guarded and transmitted in their House. Since the Stewardship had become hereditary from 1998 onwards, so the authority to use, or again to depute the use, of the Stones, was lawfully transmitted in their line, and belonged therefore fully to Denethor. ~Unfinished Tales; The Palantiri

Denethor was also a rightful viewer of the Stone (though he was much closer in proximity to Sauron than was Aragorn or Pippin)...however as we see he was unable to wrestle any sort of control over what Sauron showed him.

2) Denethor, Saruman, and Aragorn were much stronger as far as willpower than Pippin was:

"...nor had he [Sauron] any servant whose mental powers were superior to Saruman's or even Denethor's."~ibid

Therefor, I think it's highly possible, that Sauron saw exactly who Pippin was and how he looked. It certainly appears that Sauron won the exchange, as Pippin is in a complete state of shock and fear after he looked into it.