No, the redwood grove is close to the parking lot, where you can either park or be dropped off by rideshare. The ground leading up to the ceremony is fairly flat and covered with soft woodchips.
Yes, there is parking at both the ceremony and reception for a fee, however the parking is limited at the reception.
Please dress in cocktail attire and don't wear jeans. Be sure and bring a jacket or a warm coverup for the ceremony, as it may be chilly in the redwoods.
No, it is possible to get around the city without renting a car! There's a BART train that travels from both SFO and OAK airports. San Francisco also has the MUNI, which is a frequent light-rail and bus service. The ridesharing apps are quick, and you'll save a headache trying to find parking.
If you plan to use public transportation, be sure and download the Transit App on your phone for accurate time predictions and detailed system maps. You can purchase a rechargeable clipper card at the BART train Stations, Walgreens, or at the Ferry Building on the Embarcadaro, otherwise be sure and have exact change for the fare.