I took a look on mta.net and found the best way back would be aboard the No. 720 "rapid" bus running from Colorado and Ocean (maybe even Pico and Ocean, if I'm readying the bus' route map correctly) in Santa Monica all the way 6th and Central downtown. The schedule states there's an eastbound 720 bus that runs about every 10 minutes between 5:30 p.m. and midnight and the travel time is probably somewhere around 75 minutes.
If you put out any additional updates you might want to suggest that any walkers planning on returning via the MTA park their cars in the neighborhood of Sixth and Central to minimize any additional after-the-bus walking. In other words, I'd rather park there and walk in the morning light down Central to the Coke building than park down at the Coke building in the morning and have to get off the bus and walk down there to get back to my car after dark.
But for better or worse the area around 6th and Central is mainly industrialized on the eastern fringes of skid row with street parking but little in the way of supervised lot parking, so maybe people wanting the added security of off-street parking should park at lots along 6th Street through downtown and plan to get off the bus accordingly. They should make sure the lots are open late at night though!