Re: What would happen if Saruman got the ring?
I think that some sort of confrontation would have been settled between Saruman and Sauron; maybe it would have taken a while to Saruman to master the Ring, but after that I suppose I could have find a way to somewhat force Sauron to a direct confrontation, eventually destroing him or making him a sort of gost or lesser servant. This is the reason why Sauron feared so much that someone powerful would have claimed the Ring; if he could only simply flee and somewhat plot to recover it he woulnd't have been so mad about hurring his assault on Gondor. Furthermore, as the Ring Wraights are clearly reported to be slaves of the One, he would (Saruman) have got power over their will too.
I don't think he would have got immediatly rulership over Mordor armies, except maybe those under direct control of the Nazguls, but for sure as long as their former master was overtrown, those buddies would have to decide wheter to be eradicated or join Saruman.
As for what he would have done as the new Dark Lord, well this is quite plain, considering what he did in Isengard; he would have made the world a great factory, polluting and exploting natural resources for greed and even just for fun, making himself and his armies more and more powerful, while conquering or exterminating every people that would oppose him or not worship him and surely as soon as he would have felt ready he could have tried for Aman ...
this is, of course, just my idea 
finally, regarding the other issue, I think that every Maya and the majority of the High Elves (at least those who were in Aman) would have had strength enough to master the ring to its full power; on the other hand, I don't see why someone like Denethor or Aragorn would become ghosts; it is said that the One gives power according to the "stature" of the bearer so I think, with their strong will, they would have found a way to master it, maybe not at its full power. They would probably have been corrupted by its evilness, though this is not sure too, as as far as we can see from direct sources, much depends on how you get over the ring (Gollum went the evil way because he murdered Deagol, while Bilbo was even able to surrender it to someone else!!), for sure they wouldn't be able to destroy it by their will (that was basically what happened to Isildur, he wasn't able to destroy it, but he hadn't became evil at all), so to conclude my tought I think that someone like Aragorn, being forced to go against his will and to take the ring, would have probably became a serious treat to Sauron, being able for his part to unite under his will all the free peoples (at least men) and maybe forcing also Harad or Esterling to follow his will or at least to retreat their support to Sauron, while at the same time he maybe had enough power to take the ringwraights to his side (being them just but men like himself).
"In [Sauron's] actual presence none but very few of equal stature could have hoped to withhold it .. " that's the point, ACTUAL PRESENCE, so I think Aragorn could have done much with the ring and turned the tide to his favour before there will be (if ever there would be) a direct confrontation with Sauron, which I mean something phisical, not just tring to master the ring. I really doubt such a confrontation would have been risked by Sauron on his own will (otherwise he would have set forth by himself to recover the One), because the most part of its power was in the ring, so he knew he would have risked to expose himself to be destroyed or at least enslaved if the ring was in the had of someone powerful enough to master it.
just a bouch of tought, I don't have the heart for working hard today