I learned a long time ago to walk through the stores and shade-hop my way through the parks. I feel that in places Epcot is harder to find shade in, and AK feels much hotter. I can deal with MK just fine. We just avoid the hub all together as it's usually jammed with people and other than parades and shows, neither of which we're into at this time, there's no reason to face that.
Be careful when you generalize about people's medical (and mental) conditions. There are some who are in poor shape to start with and the humidity combined with the heat catches them off guard. And, just as there are helicopter parents who are diligent about taking care of their child, spraying them down and making sure they drink, I've seen many parents oblivious to a sunburned child as they watch the afternoon shows out in the hub. Perhaps trees would help them, probably not... but they did offer a visual oasis to make folk consider taking a break.