My own experience -

One does not need to call in to request.
One can make the request online if one has the ticket/ pnr number and name of passenger.
Calling in is usually to reassure oneself that this has been done.

1) Yes. No need for a doctor's certificate.
But they should remind agents when they check in at the airport and at the gate.

2) At BOM/ Bombay/Mumbai it is very efficient. The Delta folks are available outside when one alights from he car/ taxi, one can get the wheelchair assistance.

3) Yes - call AF as well and make wheelchair request, but again one can make this request online at www.delta.com and at www.airfrance.com

4) Yes - most likely last off if they want help unless they just get up and walk off on their own. Usually the wheelchair folks are just outside in the aerobridge.
But they should remind the flight crew atleast 30 minutes before landing at a USA airport.
Having assistance makes going through immigration + customs easy. And at USA airports the wheelchair assistance agents bring the pax & baggage outside to waiting relatives.