I agree that your schedule is far to planned out and regimented. Nothing will ever go exactly as planned. So I think you will be stressed out more when the plan doesn't happen.
Even with the closures, I'd still recommend to slow down and do a full day at each park. Especially since you are all first timers. Each park will have plenty to do since you haven't seen any of it before. Each park is great in its own way. And if you feel you are done at a specific park, then make your decision to hop to MK to get extra time there.
Regarding it's tough to be a bug, I think it will be too scary for the 3yo for sure and possibly for the 5yo.
At Epcot, the kids will enjoy Finding Nemo, The Seas pavilion, Soarin', mine liked Living with the Land, and Test Track. And then there are different attractions and entertainment in many of the countries and the pavilions themselves. It is very easy to spend a full day at Epcot exploring.
I think your DHS day would be the easiest to cut out early and head to MK.