About the number of Saruman's forces:
While it seems reasonable to think the hobbits understated the numbers (I'd picture 50,000 plus) the 10,000+ figure seems, in the context, fairly authoratative. And it's interesting to note how others fasten on that 10,000
Merry estimate (the movies and also Forstad's atlas). Furthermore, a good case can be made for Saruman having c. 10,000 or so: the limited pool of dunlendings
to draw on, the limited geographical extent of Isengard, the need to keep a form
of secrecy (remember Gandalf being bamboozled). If so, then Saruman did, as
noted above, play a remarkably dangerous game, but , as Gandalf observed,
Sauman's one real chance was to get the Ring. With that, and a strong base and devoted and highly trained army centered on Isengard, he was in a strong position, otherwise---he's toast
I really find UT fascinating, especially, the Battles of the Fords of Isen. So, if
as JRRT opines, Rohan's defense failed conceptually, what should it have done. Given that perhaps the generally preeminent role of cavalry in history is scouting and harassing the enemy, interdicting enemy movement, etc., should Theodred have positioned early warning forces both near Isengard and about the area of the fords, with infantry preparing Helm's Deep?