You could definitely pull this off, but it is going to be a long day and if you are parking in the garage / lot, I am not sure if you have in / out privileges if you did want to go back to the hotel to freshen up.
We never went back to the park after we left during the one and only time we stayed off-site.
You may want to compare your off-site room rate to the Cabana Bay when it opens this Spring. They don't offer complimentary Express, but the rooms are much more affordable and you could definitely walk to the hotel and back to the parks in about 10 minutes total. Not sure about hotel parking fees yet, but I am sure they will be the same as the parking structure or a couple dollars more per day.
I think that would be a smart move and it would also give you the same freedom as staying on-site, just a lot less expensive than the other three hotels.
Yes, it's feasible to go the parks and stay for the nightlife at CityWalk without traveling back to the hotel in-between.