[Boromir88]

If that's the case, would Sauron think that a hobbit could seriously contend with his own will?

All the while, Pippin falls like a deck of cards.

We actually don't see the effects looking into the palantir and the mind to mind transfer Pippin had with Sauron until after the fact. Pippin seems fairly composed when he gets Sauron's message, although he is filled with fear, he is able to remember what Sauron told him to say and he does answer Sauron's question of who he is. He doesn't show the effects until after the use of the Palantir.

Aragorn is able to resist Sauron in his wrestle through the palantir

Comparing Aragorn's will and ability to wrestle away control from Sauron through the palantir to Pippin's use of it just isn't fair. Aragorn was 1. the rightful user of the Palantir and 2. the farther away the palantir is from Barad-dur the weaker Sauron's will is.

Gandalf says that those with a will of adamant can resist the lure of the palantir towards Barad-dur

I don't think Frodo had such a will. Frodo put on the Ring several times (which seems to me that he didn't have a will of adamant since he couldn't resist the desire to put on the ring). So he very well could have found an urge to look into the palantir had he ever come across one. If we are to even take Gandalf's words as truth, did Frodo have such a will that he could resist any sort of urges? I doubt it, as again he did put on the Ring on several occasions, but more importantly did Sauron believe it?

and, according to the source quoted above, the witch-king had more fear of Frodo than of Aragorn.

Your mis-use of the quote is simply astounding. It doesn't say the Witch-King feared Frodo, because he was Frodo. The Witch-King feared Frodo because Frodo had a blade that could essentially destroy him:

Escaping a wound that would have been as deadly to him as the Mordor -knife to Frodo (as was proved at the end), he withdrew and hid for a while, out of doubt and fear both of Aragorn and especially of Frodo. But fear of Sauron , and the forces of Sauron's will was the stronger.

The Witch-King didn't fear Frodo because he felt intimidated by Frodo's physical traits, or any other reason other than the fact that Frodo carried a blade that could cause his end, and indeed did end up causing his end.