- You are right. Saruman should have seen the Ents. He should have added them to his plans. Failing to do so was a big, huge, massive mistake. And he falls partly because he did not take account of them. So I wholly and completely agree with you there.
- Theoden's recovery and everything that happens because of it; the muster at Edoras, their arrival at the Hornburg ext. ext, all has one source and that is Gandalf. Gandalf's return from death was unforeseeable. No one expected that, nor can we blame them for not doing so.
- As for the events after that, the Rohirrim leaving Edoras ext. Saruman is not omniscient, he does not know everything. So there are really only two ways he could have known that Theoden had been restored and was now riding west.
- The palantír. We know that Saruman had one, and that he had used it in the past. So it is possible that he could have seen the events in Edoras shortly after they had happened. There are some problems with this idea though. First the palantíri seem to be under Sauron's control. But the bigger issue is that Tolkien never specificity states, unless I have forgot about it and it has been a while since I last ready the History of Middle-Earth and some of his other works, that Saruman ever saw anything like this, or that we even used the palantír at that moment. So we move on to the more probable way Saruman would have learned of those events.
- Grima. Grima would have brought word of what happened when he fled Edoras. The issue here though is that he arrives after the Ents have all but destroyed Isenguard, and thus after Saruman's armies had left.
So what conclusions can we come to based on this? A few I think.
- When Saruman sends out his armies for all he knows Theoden is still his puppet in Edoras. Saruman's armies can sweep across the Westfold either destroying or taking the Hornburg and then make a full attack on Edoras to completely wipe out Rohan.
- The knowledge that Theoden is in fact well again and on his way west comes to late for Saruman to with hold his armies.
- Saruman though not knowing of Theoden makes a huge mistake by leaving the Ents out of his plans.
- Conclusion. Saruman's fall was as I said before a combination of his own lack of foresight and the intervention of "Fate"