You're worrying too much about the lines. I would never go to WDW this time of year in any year. I would dream about going in mid-Feb but my kids are always in school. Lines really should not be an issue.
The sun will be setting as the parks close, so if you get in line for Splash at park closing, you're going to be able to see MK at dark. Same thing with a safari at AK just before close. Alternately, you can schedule a restaurant reservation just before park closing and the parks will be vacant when you're leaving.
My young kids would miss the meet-and-greets at this age, so that's a consideration, but you can do that in CA.
In Epcot - take time to explore the countries. They are really well designed. And we didn't do Mission:Space for years, but it quickly became a favorite for my 6 and 10yo. Might be too much for a 5yo but you will probably enjoy it (unless you're claustrophobic/easily nauseated).
Check the thread for Rise of the Resistance, but I believe that you should be able to get a reservation from your room. When we were there in October, you couldn't so the park was uber crowded at rope-drop so we did MMRR as soon as we got there, but then took the skyliner back to our resort to avoid the crowds. We returned for our 1PM Brown Derby reservation and after a nice relaxing lunch and their margaritas, it was an amazing afternoon. I don't know how the crowds will be at HS these days but don't fight them. If it's crowded, give it some time and try again later.
WDW is not really best if you try and rush it. Take your time and enjoy the vacation!