There's no real public transportation to/fro Marin to San Francisco, so how do you add substantial units to Marin without greatly impacting traffic which is already pretty painful?

The same NIMBYs that prevent new housing also prevent new public transit. Build both - whichever one comes first will speed the other along.

BART was intended to go there when they first started building it, iirc. Marin dropped out of BART, and then San Mateo county.

This is why there's the double decker tunnels downtown - one was intended to branch off towards Marin.

>There's no real public transportation to/fro Marin to San Francisco

After the loans on the construction of the Golden Gate Bridge were paid off (some time in the 1960s or 1970s) instead of eliminating the tolls, it was decided to use the revenue from the tolls to subsidize public transportation between SF and the counties to the north of it. The buses and ferries between Marin and SF remain very good IMHO.