As to the broad gauge etc:

IMO, broad gauge was to discourage use of existing subway-car designs. Given the state of subway-car designs then available, I think BART did the right thing to insist on all-new designs. For instance, PCC-based designs, which were great for light-rail purposes, don't strike me as adequate for a system like BART. However, I do concede that the new designs had a lot of bugs which took a long time to fix.

The broad gauge also allows more efficient use of the circular space made when boring tunnels, and for the Transbay Tube.

Still another advantage of broad gauge. There is not a single grade crossing of a public road, anywhere on BART. Suppose BART had tried to run on train tracks that already existed. Those usually had grade crossings. To contrast: Think of all the grade-crossing accidents that Caltrain & its ancestors have had. As costs rose, there would have been excessive political pressure to abandon the complete grade separation.

BART also uses 1000 volts DC, which allows for lighter-weight motors etc. than the 600 or so volts that was pretty much standard. That has saved BART big bucks in electricity over the years. Many interurbans used 1500 volts or more; but such voltages require more space in tunnels, which raises construction costs. Those interurbans didn't have the large mileage of tunnel that BART has, and they had lots of grade crossings.