I think you have the rest days in the right spot. I am with Alex on wanting to be in the parks everyday but DH has made me take breaks. I am very appreciative after because I really needed the break.
I'll admit, it does feel like you're missing out on the fun during rest days, but the heat and the walking really does a number on you. Plus, being able to sleep in without an alarm is invaluable.
Awesome! It's great that you get to go back while the kids can experience more of it and are excited about HP.
I do worry that once Landon sees all the HP stuff, he's going to want to skip Disney next trip. Hopefully, the snacks at Disney will keep him around. His current world revolves around his stomach.
I love your plans and your description of them!

I'd be surprised if the days didn't play out like I described them. The struggle of traveling with multiple sets of touring styles.
The cuteness in this picture!
Hard to remember the kids being that small. Seems like a lifetime ago!
Reading HP with your littles is such a magical experience. My boys adored those books and it was so fun to read them together.
We've been having so much fun reading the series. Reading before bed has been our nightly ritual since they were born, and while the books have changed (we've gone from Dr. Seuss to Roald Dahl and eventually Landon wants the Hobbit) I appreciate the time spent together. I know one day it's going to end, so I'm soaking it up while I can.
You will have the best time touring HP land in Universal.
Watching the magic through the kids' eyes is going to be something special. I've told them basic information, like the attractions and foods, but have tried to keep the smaller details a surprise. For instance, Landon just learned about the Weasley's joke shop and has no idea there's actually a shop in Universal modeled after it. He's going to be so excited to discover it.
I never stay for the whole day. I usually go for 3-4 hours, take a break and go back later.
Those midday breaks are essential! It can really help to decompress for a few hours before diving back into the hustle and bustle.
My goal is next trip to have a longer stay so we can work in a rest day. Or at least a no park day.
It's hard to feel like you're "wasting time" on a Disney trip, but sometimes it's just the thing you need to push through the remainder of a trip. I enjoy my 4 and 5 days trips where I'm go, go, go. But anything longer and I need a rest.