It is true that the 5K is a bit expensive compared to other 5K events around the country but running or walking in a race in Disney is like no other.5K is a race I can get behind. I've even "run" in a 5K! (I use the words "run" in quotes because it was more of a fast-paced walk/jog. I've never been able to run worth a damn my whole life, and now I'm old and creaky - well middle-age and creaky.) If it wasn't so expensive to sign up for just the 5K, that might be worth checking out!
Sounds like a rather busy day moving from the race to MK to another race. Don't wear out your poor Grandma! Otherwise it sounds like a great plan. Hopefully you will have no problems getting your buses back to make your Splash Mountain FP! (Use the train when you get back to MK to get to Splash. It'll likely be faster than walking through the park - first stop! - and easier on tired feet!)
Good idea about the train I didn't think of that.