Taking it from the top:
geddes wrote:- General advice for taking the coast starlight (unfortunately I can't afford a sleeper car) is there a way to get a shower in coach?
No. Showers are for sleeping car patrons only.
- I want to stop in San Francisco for a couple days, but it looks like the Coast Starlight doesn't actually stop in the city. Oakland is as close as it gets. What is the best way to get from Oakland to San Francisco Proper?
There is a connecting bus to SF. Or, detrain in Emeryville and ride a San Joaquins train one stop to Richmond and ride BART under the bay into the city.
- Can I buy one ticket from Portland to LA and just stop in SFO and use the same ticket a couple days later to finish the journey?
No. The Starlight is an all reserved train requiring a separate ticket for each leg of the trip
- Why does it take 12 hours to get from Oakland to Los Angeles on the coast starlight? It's only a 6 hour drive - Isn't it a direct shot!
The train follows the coast and encounters numerous curves and grades.
- Amtrak offers a 8 hour way to get from San Francisco to LA, but it involves two buses. A bus to Stockton, a train to Bakersfield, and a Bus to LA. How nice are the busses, and why is this indrect route so much faster than the nice coast starline?
I don't do busses so cannot help here. The "indirect" route is better engineered than the direct route.
Otherwise, enjoy the trip.