What if Isengard joined forces with Gondor?

[Raynor]

I guess Saruman did promise service to Sauron but did he ever really mean it?

Indeed. In The Palantir chapter, Gandalf talks about Saruman's likely fall:

Easy it is now to guess how quickly the roving eye of Saruman was trapped and held; and how ever since he has been persuaded from afar, and daunted when persuasion would not serve. The biter bit, the hawk under the eagle's foot, the spider in a steel web! How long, I wonder, has he been constrained to come often to his glass for inspection and instruction, and the Orthanc-stone so bent towards Barad-dûr that, if any save a will of adamant now looks into it, it will bear his mind and sight swiftly thither?

and earlier, at the council of Elrond, he talks about Saruman's plans:

This then is one choice before you. Before us. We may join with that Power. It would be wise, Gandalf. There is hope that way. Its victory is at hand; and there will be rich reward for those that aided it. As the Power grows, its proved friends will also grow; and the Wise, such as you and I, may with patience come at last to direct its courses, to control it.