I always heard it was an attempt to keep anyone using the rail for carrying cargo or other comercial traffic. They had concerns folks would sabotage their growth in order to resell the lines later.
You can take a ferry and then SMART all the way to San Bruno (I think they are planning an extension to Cloverdale, and eventually to Fort Ross). Personally I would rather want faster muni buses to the Avenues (or even the Persido) over BART to Marin, although People living there might disagree.
Technically yes, in practice not really, unless you're proposing a ridiculously wide gauge (several meters, or something like that!).
Even standard gauge trams have no difficulties navigating curves that are much tighter (25 m, down to 10-ish m on some legacy systems) than anything you'd seriously consider when building on a new rapid transit type rail system (normally at the very least a three-digit metre figure, ideally something more like 200 or 300 m).
The only thing that directly scales with the track gauge is the length difference between the outer and inner rail in a curve and the minimum radius down to which the self-steering effect of conical wheels can still compensate this length difference. This means that with a larger track gauge you might get a little more wear and tear for the same curve radius, but a moderate widening of track gauge (e.g. BART is approx. 16 % wider than standard gauge) will equally not cause a dramatic effect here.
In the best case, with standard gauge self-steering might work down to 150 m, which in the case of BART's track gauge would turn into 175 m, which is still rather on the tight side for a rapid transit-style line (and as originally built, BART didn't use conical wheels anyway, so they didn't benefit from self steering in curves regardless of radius used).
The bigger factor in terms of curve radii is the length of your vehicles, how much vehicle throw you're prepared to accept in tight curves, and possibly whether you want to have trains with open gangways, so passengers are able to move between individual vehicles.
