I took a look at this. Perhaps I am doing something wrong, but I am finding higher rates on the AP calendar than I was getting just trying to go in the normal way. For example, I found the Grand for $409 per night but when I went in to look at the AP calendar it showed $440 per night. Also, the AP calendar didn't have the 27th available for either DLH or PPH, but the regular calendar did. I had read that AP hotel discounts are becoming a thing of the past. There used to be a 10% discount available all the time. Not so anymore. It sounds like there are still some deals that show up for specific date ranges, but I don't think people have been finding much anymore.
Also, I just want to point out that April 26 and May 2 are blocked out on the So. Cal Select Annual Passport. I'm guessing those might be travel days for your family and your in-laws might not be planning to visit the park anyway, but thought I would mention it just in case it mattered.
Your in-laws would be eligible for dining and merchandise discounts if they had APs.
Also, it doesn't sound like it's on your list for consideration, but if you decide to stay offsite, many of the hotels and motels in the area offer AP discounts as well. Examples include Hojo, Candy Cane Inn, Carousel, Desert Palms Inn and Suites, and the Annabella. There are others I'm not thinking of right now.