LOL... ambitious, yes, but what are your other options for a working family with, at most, 4 weeks? It's 30 hours of driving to get out to the Grand Canyon (and that's in a car, so figure 35). If we average 5 hours a day we can make it in a week. So it takes a week to get out there and a week to get back. So we have 10-12 days between the Grand Canyon and southern Utah.
Of course I'd love to stay everywhere longer! And I've taken many notes and made many stars on my google map so that when my wife and I have months in the future we can do all of the cool stuff I'm seeing.
But we have to do our best to make our memories in the time constraints we have.
As far as backtracking, the fastest way home from Moab is on I-70 through Colorado and Kansas. It's not backtracking. When I say "do Colorado" I mean stay there for a night (at most 2), not see the whole state. Plus, by that point in the trip the kids will be "natured out" and just want to go to an amusement park or the movies.