I'll echo everyone else and say ditch the ADR, or move it up to 3-4 in the afternoon so you can be ready when the party begins. The Halloween stuff is only out for a few hours and you really don't want to spend that time in a restaurant, trust us.
The Horseman is awesome. He rides 5-10 minutes prior to the parade. For the animal and rider's safety, expect the CMs to be extremely strict (even more so than usual) about keeping you and your kids out of the route in advance of his ride.
Also, know that it doesn't take much inclement weather for Disney to cancel the Horseman's ride, the Parade, or both. It isn't safe for the horse if there is any wet pavement. This particular parade is also much heavier on CM performers than any of the other Disney parades - meaning there are large groups of dancers on foot (from the grave diggers to the ghostly dancers and so many others). Again, for their safety, it doesn't take much rain for them to cancel the parade.
We have been to the party 5 times over the last decade, and we have seen the Horseman 2 of those times and Parade 3 of those times (the other times either the Horseman alone or both Horseman and Parade were cancelled due to weather). So if you are hyping it to kids, be sure they know that these events may be cancelled, to avoid tears.
On the bright side, the castle light show and fireworks seem to go on no matter how rainy it is. Last time we went, a couple of years ago, it poured all night, and they still did those. The candy and character signing lines never seem to be affected, as they put the candy barrels under shelter. I cannot recall if they did the Villain stage show that night or not.
Wait times at rides will be almost nothing, because many people crowd and camp out in the Hub and Main Street to see the parades, fireworks and stage show, and another huge chunk of them spend the entire night in character lines. Know your park shortcuts to get around without having to cross the Hub (you'll also often find candy station along these back sidewalks). We usually ride and trick or treat our way around the park, and walk onto pretty much anything we choose to ride.