If he was, what would be holding the Nazgul to his will?

The Elves were able to be free of this hold initially by taking the rings off. The bearers of the One were all said to be free of it's 'influence' when it was no longer theirs.

The Elves were able to perceive Sauron's thought whenever he first put on the One, the Nine were not, since Sauron was involved in their making.

When you consider what the rings were, they were intended to be used to gain control over the races in ME, by controlling their leaders, who were wearing rings of power, and ultimately, through the One.
I think this 'link' was *only* through the Rings, and that without the other rings of power, the need of Sauron for the One would only be to recover all of his own power.

Apparently, Sauron wearing the One ring was *slightly* more powerful than Sauron before he made it, although the real issue in LOTR, in my opinion is Sauron potentially getting back to full strength, while the peoples of Middle Earth were diminished in power and ability, and the consequences of the bearers of the Elven Rings not perceiving and reacting to Sauron quickly enough (two, at least were different bearers from the time Sauron first put on the One) if he put on his ring.

Sauron 'gathering' the Nine, I think is just him recalling the Nazgul to himself, to be his main servants, instead of using them for their original purpose i.e. to control the people over whom they were kings.

In my opinion, the One Ring does not 'think' for itself, nor did it 'awaken', or to see a bearer of the one from afar (if you're in any way interested in my thoughts (this, and the  reply to Curious) on invisibility and stuff, I made it up only yesterday! :) ).
It is my opinion that Sauron was only starting to try to perceive the thoughts of any bearer of the One, after he learned it was found (and ultimately gain knowledge of where they are or to deceive their thoughts into revealing themselves), by using this sort of 'link' between the Rings, since the One was designed for this purpose. Unfortunately for him, the three hobbits who did wear it were not able to understand how to use it, although Bilbo and Frodo, at least, thought they were able to perceive an eye looking at/for them.

Since he was unable to do this as effectively as he would if he had the One himself, it is my opinion that he was likely to be using one of the lesser Dwarf rings for this purpose, which is why, I think, he was able to keep control over the weaker Nine, and very nearly got into the mind of Frodo, when he wore the One on Amon Hen, before Gandalf, a being of equal stature as Sauron (if not physically, as an Istar, at least mentally), wearing a ring more powerful than the dwarf rings was able to 'block'(?) Sauron and allow Frodo to regain his thought before any damage was done.

Of course, this is just my opinion, and it is probable that I'm the only one who thinks it possible. :)

I am also not trying to deny any influence from divine beings here, but I do think that divine influence was involved in the creation of the rings, although I think in this case, it was most likely limited to the two Holy Ones (Gandalf & Sauron) directly involved. The influence of the Valar/Eru being much more subtle...

"... in storm he delights, and laughs amid the roaring of the waves."