As Book!LotR fans are likely aware, Saruman was the same sort of being as Gandalf (i.e., a Istar and almost certainly a Maia) who was sent to Middle-earth with the same mission Mithrandir: opposing Sauron, not through direct power but by wisdom and inspiration of Men and Elves. Like Gandalf, Saruman presumably began his career in the Mortal Lands as a genuinely good being, though of course deprived of much of his angelic knowledge and power. He did not create build or even design Orthanc himself; that tower was built by the men of Westernesse, and Saruman was given leave to dwell there by a Steward of Gondor whose name I am too lazy too look up. It is said that Saruman petitioned leave to dwell in Isengard because he knew a palantír was there (presumably forgotten by Gondor) and that in attempting to use it for his own purposes, he fell under Sauron’s influence and became corrupt.
Which brings me to the thread question. Suppose the seeing-stone hadn’t been there? It could easily have been taken by one of the Gondorians when the abandoned the tower, or stolen by a Dunlending, or found by a passing Orc or Nazgul during the time of its vacancy and taken to the Dark Lord. The specifics don’t matter, at least not to me, except as it leads to the thead question: without the palantír, would Saruman have fallen?
We don’t have much text to go on to support any speculation, but I think Saruman desired power from the moment of his arrival in Middle Earth as one of Istari. I think the corruption would have appeared sooner or later regardless, because of his basic inclination to use his power for personal gain.
I think he would have been corrupted, but probably not in the same way.
Remember that Saruman was not the exception among the Istari; all of them went off-mission except Gandalf, and were “corrupted” in their various ways. I suspect that without the palantir, Saruman would have gone the way of Radagast, in his own fashion. Something in his nature drives him to seek order and knowledge, and I suspect his request for Orthanc was an expression of that: a place he could order to his liking, and pursue knowledge, free from the interference and petty problems of mortals. The palantir was part of that, but I think wanting it was a symptom. For that matter, I think his desire for power was an outgrowth of his desire for order; without Sauron pushing him, I don’t know that it would have gone so far.
I would guess that the end result would have been a Saruman that wasn’t really evil, but was careless of others and ultimately pretty useless. He would try to hold onto his isolation as much as possible, avoiding involvement, or even refusing to acknowledge what was going on outside. At best, he might provide some useful information.
An isolationist Saruman would seem to clearly be better for the good guys than an evil one, but he would still have been corrupted. Also, without his interference at various points, events would have played out rather differently–it would have freed up Rohan to intervene earlier and with greater force, but (IIRC) it would also have changed the breaking of the Fellowship and prevented some distractions and misinformation from affecting Sauron. Would it have worked out better, or worse?
In this scenario, isn’t it likely that Gandalf would have counseled that Frodo and the Ring come to Saruman for advice (individually or at a session of the White Council), either instead of or after the meeting of the Fellowship at Rivendell? Saruman would have been faced with Galadriel’s test, and from what we know of him, likely would have failed the test and attempted to use the Ring against Sauron.