I just have to ask. You are doing a 27 hour drive, 7 days with a theme park every day, 27 hour drive home, all over the 4th of July holiday with MIL in tow?
Unless you are used to this pace and that much togetherness, I would highly recommend seeking some general touring advice on this board. This is a pace that would kill my family after about 4 days, if we lasted that long (my kids are 6 and 9). Most everyone needs downtime. The parks are massive. Consider that you may be so tired by day 6 or 7 that at the very least you should be flexible rather than committed to a park. More likely you will need a veg out day (or afternoons) sprinkled throughout. Also, being 27 hours away from home on 7/1 suggests you aren't accustomed to the way that Florida heat and sun can zap your energy. Go prepared for a slower pace. If you are staying onsite, carve out some down time to enjoy that expensive resort you have paid for.
Back to Epcot- I agree that the Living Seas does not compare to Sea World. It has some nice features, but by comparison it is miniscule. My not-thrill-seekers do not love Soarin and will not try Mission Space or Test Track. That might work in favor of your schedule if you are trying to whittle things down to manageable. Sum of All Thrills would also be a no-go.
To the poster who suggested we haven't done both - my kids were the ones begging to get to Epcot, and they truly enjoyed a very full Sea World day in San Diego. I don't know if I can pick one over the other, though we've done both.
MIL and FIL are not driving with us, they're flying down, staying with us (though it's not an expensive resort, we've booked a house at Windsor Hills for a fraction of the price of those "expensive resorts", lol) then flying home. We travel with them for a week every summer. They're phenomenal to travel with.
Surprisingly enough, even though we're from Canada, we're very used to high humidity. In the summer, we typically have humidity levels of 80-100% every day in July. Granted, our normal temps are 85-90F, but, we get the high humidity. And we normally always get a heat wave at some point of 95-100. Welcome to Atlantic Canada - home of temperature extremes: 90F with 90% humidity in July, and -40F with 40 mph winds in January, lol.
We're planning a slow pace. Yes, we're planning a park every day, but we are NOT planning on being there from open to close. We're just not those kind of people. We will either likely do a 9-4 type thing, and plan to focus on indoor shows etc... in the afternoons and have our evenings to relax with normal routine such as supper, bedtime etc... or we'll do a 9-1, break for the afternoon, and go back for the evening type thing. Also, we've strategically planned Discovery Cove in the middle of our vacation as a "down" day, since it's so much more relaxed than WDW. We're also taking an entire day's break when we arrive. We'll be driving 3 days, and then our first full day in Florida, we are doing nothing but relaxing, and then going to Hoop dee Doo that evening.
But, I do appreciate the concern, and certainly will take advice/input. And we are definitely open to adding a day.