It is A Horse and His Boy, not "The Horse and Its Boy". I suppose by controversial, you mean the "Muslims"? I honestly don't think they'll have a problem, they'll mix them up just enough to break the obvious analogy to Muslims found in the book. Wouldn't be hard to do at all.
However, I highly doubt A Horse and His Boy will ever get made. Each new movie makes less than the last, Prince Caspian was pretty awful, it was downright amazing that Walden Media pressed through and made the third film after how poorly the second one did (Demestically it didn't even break even, it made some on international sales, but that is a volatile market and a risky business venture). Despite The Voyage of the Dawn Treader being my favorite of the books, I never bothered to see the film since I was warned that it doesn't even loosely follow the book's plot, apart from being on a ship.
It has been confirmed that the next Narnia film will be The Magicians Nephew instead of The Silver Chair. Why? Because they looked at book sales and realized that next to The Lion, the Witch, and the Wardrobe, the second highest selling book in the series is The Magician's Nephew.
When it comes to a series of books, I am not sure sales account for popularity (in fact, in this case I am certain they don't). Even though The Voyage of the Dawn Treader is my favorite book, I've never decided to just go out and buy a copy of it alone, or give a copy of it as a gift. I have given several sets of the series as gifts. If I have ever given just one book from the series as a gift, it has been The Lion the Witch and the Wardrobe. Why? Because it is the first book in the series. The fact that they republished the series in chronological order after Lewis died made The Magician's Nephew (always my least favorite of the series) book one. Imagine someone recommends a book series to you, you think it sounds interesting so the next time you are in a bookstore you immediately look for the first part of the series. Of course The Magician's Nephew's is the best selling book; for the last few decades it has been the first book in the series. I wonder how many people bought it and never got around to reading it. Or bought it, read it or started reading it and decided it wasn't for them.
It is obvious they are getting desperate here by abandoning any logical order and now basing which one comes next on book sales. I have a feeling after The Magician's Nephew flops (and it will), we can fully expect it to be the last film in the series.