Oh yeah, one more place to cross the Ship Canal - Ballard Locks. The actual structure at the Locks is very stable (thank you Army Corp of Engineers), and should survive even if an EQ destroys or every other structure is compromised. This is also the westernmost crossing point, and a little out of your way, so I would at least check the Fremont and Ballard Bridges before walking over to the south end of the Locks. And expect to wait in line, there's only room for 1-2 to walk across the locks side by side. The Fremont Bridge was retrofitted and updated in recent years, and as a 90+ year old bridge nothing is certain, but you can be fairly confident of using it as at least a foot bridge.