Most bar soaps aren't very suitable for hair.

There is a natural Seed brand bar shampoo. A friend found some in NYC and liked it, but she forgot where she got it.

Get a bar of Dr. Bronner's natural soap from a health food store or co-op and try it as a shampoo at home. If it works, put a new one in your kit.

Don't use aspirin as a preventative, it thins the blood and hinders coagulating. An earthquake is not the time to have clotting issues. (I mean this differently from taking your doctor's advice about an aspirin regime against heart and stroke problems.)

Some extra stuff is not a big issue. You should be stopping along the way to rest periodically, a good time to lighten the load if you need to. You can base your decisions on what IS, rather than trying to foretell WHAT MIGHT BE. Leave your discards in plain sight -- someone else may have a use for them.

You will probably end up using some of your gear for reasons totally unforeseen before the disaster. Or the disaster you planned for is not the disaster that has happened. A big quake won't have the same issues as Mt. Rainier blowing her top. Be prepared to jump in different directions.

Sue