I'm stunned that the Amtrak website offers option #3. Upon arrival at SAS, everyone on train #21 must leave the train except those in the last 2 cars, eg, train #421. While it might be possible to immediately board one of the rear #421 cars, I wouldn't count on it as the entire crew leaves the train there and based on my own observations, do so quickly once all the passengers are off. OBS crew goes to the hotel about a block away for the night. The 'new' sleeping car attendant for #421 may or may not be there yet. As a result, the passenger(s) would end up killing time in the station until boarding time, typically 10 minutes or so prior to departure.
On the other hand, as I noted in my previous post, it's possible that a passenger boarding in DAL, for example, encountered 'sold out' in the #421 section and so booked train #21 to SAS after which space was available on #421 (or even #1) from SAS to LAX or any intermediate stop between the two.