This is a great answer! I had thought it was just foreshadowing of his future betrayal. Did Saruman believe that he could use the Ring's power without being corrupted by Sauron, or did he just not care if he was?

To look at it from a different perspective: at this time, Saruman didn't want to believe that Sauron would or could return. A threat one refuses to see is a threat one is likely to fall victim to. This could be seen as part of the reason, psychologically, that Saruman was able to be turned, as opposed to Gandalf who was expecting Sauron to return before he even did, and thus was ready and waiting to resist Sauron's many corrupting influences—the One Ring itself being, of course, the most obvious.

In the version of TESB that I recall, there were other Imperial forces besides just the AT-ATs—they were simply the largest and, therefore, most prominent.