I took help of travel agents. Going to airlines directly is more expensive than buying ticket from travel agents. I am not sure if the wheel service is guaranteed but all airports are wheel chair friendly. You will get one if you choose that option. Proof of disability is not required. My dad, in fact, is not a disable. This is a kind of assisted travel. And it was free for my dad. I think it is free for elderly people. One of my friend was telling me that she sometimes sends her kids back home with assisted travel option but she had to pay something like $200 per person for kids.

Thanks