[Maédhros]

I think that the answer to this question might lie in the deception Sauron carried out during the creation of the Rings. As one part of sanwe is unwill, i.e. the ability to close the mind to perception from outside, then he would have needed to carry out some deception in order to prevent those barriers being put up; he would have needed to get the Elves’ trust.

I would rather say that when Sauron was using the One Ring and the others the lesser rings (3 Elven, 9 men, 7 dwarves, etc) that somehow the Rings made the wearers of the Ring unable to close their minds from Sauron using the One Ring.

Finally, I have another interesting thought to add about the nature of the One Ring on mortal ringbearers. Each of Gollum, Bilbo and Frodo experienced bodily decline from wearing the Ring, so each of them saw their hroa diminish and hence the ‘mantle’ which protected their thoughts. If the fea is united to the hroa in incarnates then wouldn’t this also diminish, thus explaining the mental anguish of each? What intrigues me is how Sauron was able to ‘see’ into their minds. Was this due to the fact that much of his power was invested in the One Ring, or could it be that the Palantiri were in some way created for purposes of osanwe?

I would say that the inverse is correct. While Bilbo and Frodo had the Ring, their body endured more that it's natural span, notice that Bilbo looked very similiar from his adventures in The Hobbit and he was about 111 years old.
I have explained this in another thread. The Ring puts the hröa and fëar against each other. While a mortal fëar after sometime would want to leave the world and it's hröa in time would decay, the ring makes the hröa endure more that it is meant to do, this puts great anguish against the being who has the Ring.
From Morgoth's Ring: Myths Transformed

But let us suppose that the 'blessing of Aman' was also accorded to Men.* What then? Would a great good be done to them? Their bodies would still come swiftly to full growth. In the seventh part of a year a Man could be born and become full-grown, as swiftly as in Aman a bird would hatch and fly from the nest. But then it would not wither or age but would endure in vigour and in the delight of bodily living. But what of that Man's fëa? Its nature and 'doom' could not be changed, neither by the health of Aman nor by the will of Manwë himself. Yet it is (as the Eldar hold) its nature and doom under the will of Eru that it should not endure Arda for long, but should depart and go elsewhither, returning maybe direct to Eru for another fate or purpose that is beyond the knowledge or guess of the Eldar.
Very soon then the fëa and hröa of a Man in Aman would not be united and at peace, but would be opposed, to the great pain of both. The hröa being in full vigour and joy of life would cling to the fëa, lest its departure should bring death; and against death it would revolt as would a great beast in full life either flee from the hunter or turn savagely upon him. But the fëa would be as it were in prison, becoming ever more weary of all the delights of the hröa, until they were loathsome to it, longing ever more and more to be gone, until even those matters for its thought that it received through the hröa and its senses became meaningless. The Man would not be blessed, but accursed; and he would curse the Valar and Aman and all the things of Arda. And he would not willingly leave Aman, for that would mean rapid death, and he would have to be thrust forth with violence. But if he remained in Aman, what should he come to, ere Arda were at last fulfilled and he found release? Either his fëa would be wholly dominated by the hröa, and he would become more like a beast, though one tormented within. Or else, if his fëa were strong, it would leave the hröa, Then one of two things would happen: either this would be accomplished only in hate, by violence, and the hroa, in full life, would be rent and die in sudden agony; or else the fëa would in loathing and without pity desert the hroa, and it would live on, a witless body, not even a beast but a monster, a very work of Melkor in the midst of Aman, which the Valar themselves would fain destroy.

No indeed, that would be a ridiculous assertion. I was speaking of the difference between "incarnated" and "clothed", so when I said "incarnated" what I meant was "bound to his hröa".

If Sauron himself spent 57 years in Númenórë with a hröa, wouldn't that be time enough for him to become attached to it like Melkor was? I think that it was enough, but it seems that he was able to change the form of his.

Looking back on my last post I think I greatly exagerated the difference between Melian and Sauron. Melian was certainly quite dependent on her hröa, but I still think that, because of his special circumstances, Sauron was even more dependent than she was (I am talking about after the Númenórë thing, of course).

What is the special circumstance that made Sauron more dependant on his hröa than Melian? Because he had made the One Ring. I think that the Ring gave him more flexibility so that he was able to survive longer without being fully incarnate.