Okay...I'm not a runner but I have been at the parks for the marathon and actually was a finish line volunteer for the 1/2 this past January.
I can't help with accommodations questions but I do know that there were events the Friday and Saturday of Marathon weekend. There was an expo at the Coronado Springs Convention Center and I believe that is also where registration was.
The bulk of the marathon clears parks before park opening. Epcot is the park that is primarily impacted because that is the finishing point. By noon, most of the athletes had cleared all of the parks except for Epcot. However, even with the marathon going through the park, guests are not terribly impacted once they reach the park. Of course, traffic along WDW roads is another story. If possible, I'd recommend that your family go to a park that they can get to via monorail or boat to avoid driving.
Regarding weather - it is usually quite cold at the start, we're talking low to mid-30s. As a volunteer, I was wearing denim jeans, wool socks, a t-shirt, a sweatshirt, a windbreaker, a hat and gloves. Also remember that if you are running you'll need arrive at your starting corral well before 6am and you'll be standing around for quite a while. Disposable, thin layers would be your best bet for clothing because you may not be able to retrieve items you've shed along the course. I've seen runners using plastic trash bags to keep warm.