Something I thought about after reading these posts. You'll be leaving downtown-ish Seattle overland to Lynnwood. Assume the bridges are all down. That leaves you the Locks and water. There are still lots of people living on boats at East and West Lake Union. I'm sure you'd be able to buy/barter/beg a short trip in a dingy across to Gasworks. That should get you past your most difficult restriction.

Another thing I considered is are you planning on Hwy99? Perhaps shift over to I5 instead. Wider roads, easier access with the ramps to bypass fallen overpasses, fewer structures, trees, etc that can fall onto the road to make the journey harder. Of course, that wouldn't work if there are still cars driving up the road, but if the major bridges are down the freeways will probably be barren. Not as direct, but I think there may be fewer things that would restrict your travel like collapsed buildings, powerlines, trees, etc.

Finally, other food stuffs: Tanka Bars (buffalo and cranberries and they make a spicy one too), very tasty; Trio Bars. If you're just trying to cover the distance you can get Gu cubes or tubes like runners or cyclists use. I've also found single serve peanut butter squeeze packets at local stores (Central Market) that I've added into my pocket food menus.