Crissy Field and Fort Point are in San Francisco. I recommend parking at the Warming Hut / West Bluff Picnic Area for Crissy Field or Fort Point parking area. There is also parking at Fort Mason (East of Crissy Field) and at the Marina / East Picnic Area of Crissy Field.
Crissy Field is open year-round. Fort Point is open Friday through Sunday (although you can walk to it any time).
None.
All main trails are wide and flat, constructed of fine gravel or paved, so are suitable for wheelchairs. The Warming Hut and Crissy Field Center are fully handicapped-accessible, and both have bathrooms.