the answers to all of those questions is google.
i'm not sure if you've tried to look all of that up yourself,
the reason they used wheelchairs is because they were already set-up to wheel any runners needing medical assistance to the finish line medic tent, which was staffed with top level doctors and triage equipment. as in any marathon, you can't just roll an ambulance up to the finish line if a runner needs assistance, so they have the volunteers with wheelchairs.
here are some links to first hand accounts from the tent
http://www.wjtv.com/story/22008610/triage
http://www.people.com/people/article...693740,00.html
here is a bird's eye view of the finish line and the finish line medical tent
here is the inside of the tent
here is jeff bauman being wheeled to the medical tent by a volunteer
