Where did I suggest they walk to BART? There are cabs at all the BART stations I mentioned -- I assumed that is how they would get to/from from the station. Would cost of fraction of the parking fee for a car in SF.

It's Golden Gate National Cemetery -- it's really not very scary - quite beautiful in fact.

I've stayed in San Francisco many times -- for as much as a month at a time. Often I've had a car as we were driving somewhere else -- but even then it generally stayed in the garage while we used public transportation, unless we were driving over to Sausalito or somewhere out of town.
A few weeks ago we stayed in San Francisco for two weeks and never once rented a car (until the day we left). It was fine. We even found that taking the ferry over to Sausalito was a whole lot neater than all those times we drove over.

By the way -- we usually don't stay downtown near Union Square either.