I can't speak to how "easy" it is, but it certainly seems to be possible, as indicated from the Amtrak memo in your earlier post, which clearly indicates that they will be turning the train in Eugene at least once.

And I'm not sure I agree with your characterization of how the Coast Starlight is used along the Cascades corridor. This isn't like the long-distance trains that run along the NEC and provide no real value to local NEC passengers.

Let's leave aside the northbound CS for the moment. As you correctly indicate, it is not available for local Cascades passengers due to its chronic lateness. (Ironically, the northbound frequency is effectively already protected with a Thruway bus for the EUG-PDX segment, which runs nearly simultaneous to the n/b CS, were it ever on time)

However, the s/b CS from SEA to EUG is used heavily by "local" pax. With Train 11 off the boards, people traveling SEA-PDX will have to leave 2 hours earlier or 90 minutes later. That's not a huge inconvenience, I guess. But for people traveling between PDX and EUG (and points between), that means the first southbound train of the day isn't until 6:15 pm. It also means there will be a 5 hr, 15 minute gap between s/b Thruway bus departures. Gaps that big kind of stretch the definition of "corridor". I realize overall demand on the Cascades corridor is nowhere near the demand in the NEC, but nowhere in the Amtrak system is the word "corridor" used when there are scheduled gaps that large.

I have used Train 11 for s/b "local" travel in the SEA-EUG section. Lots of people disembark Train 11 in places like Longview, Salem, Eugene, etc. This suggests to me there is demand for the "local" service that will be lost. Again, it is a temporary loss. But I think a customer-oriented Amtrak would provide, at the very least, a bus connection.