I forwarded your post to a rail-rider buddy of mine and here is his response (FWIW):
BART and Amtrak share the same station in Richmond. Don't bother with Oakland - not near each other.
I have done the transfer at Richmond several times. You have to go downstairs from BART (elevated station) and then back up a walkway to get to the Amtrak platform. Takes about 5 minutes max.