I probably have more questions than answers, but I think there are things that need to be considered. Is your solar charger for the rechargeable 123A cells the solar charging case for the steripen? If it is, it can take 2 or 3 days to fully charge a set of batteries. While that may be just good enough for your usage of the steripen, it won't work if you need the same batteries for a headlight. Very few headlights take rechargeable 123A cells, and even if they did, the energy density is lousy for such a long trip. You're bringing a GPS that runs on AAs. This will most likely require the most power. Do you have a solar charger for the GPS? There are actually great folding solar chargers for AAs that only weigh about 4 ounces, but one will cost you around $75 to $100. I like the Powerfilm Foldable 4 AA solar charger. With included battery adapters, it will charge AAAs as well. The ability to charge AAAs opens up many more options. Just remember that it charges in sets of 2 or 4 only. The Black Diamond Storm would work well with this setup giving you a bright, waterproof headlight that is dimmable. If you want to eliminate the steripen altogether, the Sawyer Squeeze filter is a great choice. It weighs less, at 3 ounces, and isn't dependent on batteries to operate. However, with both systems, you need something different in the cold. The steripen will be limited by the batteries failing below a certain temperature and the Squeeze filter can be ruined if water freezes inside it. What stove are you bringing? Boiling water or melting snow may be required to purify water when your purifier doesn't work in the cold. If you are depending on an alcohol stove such as the Vargo Triad XE, you won't be able to use alcohol if it gets cold. Some stoves such as the Triad XE will use fuel tabs as well. Be careful in bear country if you use Esbit tablets as they smell like cooking fish. If you're using a butane stove, which I would suggest, have one that can handle the premium MSR fuel for cold weather. The MSR Pocket Rocket is a great stove and doesn't require an extra windscreen that doubles the weight like the light Snowpeak stoves. I've probably covered much more than you wanted, but for backpacking, you need to think about limiting the weight of the gear, using gear that serves multiple purposes, and having a backup plan in case a piece of gear fails.
Also, don't rule out multiple systems on such a long trip. You may want to have family send packages of consumables (food, fuel, batteries) along the way as well as tent/tarp, stove, etc. For desert sections, you'll want to pack light and bring lots of water. You might be fine with a tarp and bug bivy as shelter, and an alcohol stove for cooking. A pump filter may or may not be required to get small quantities of water into a bottle. In the mountains, you may want a tarp, tent, or 4 season tent and a butane or white gas stove depending on weather conditions. You might need snow shoes, crampons, or an ice ax. Make sure that whatever headlight you bring can take lithium batteries for the cold as well as rechargeables or alkalines for the rest of the time. For such a long trip, you'll find that no one piece of gear will be ideal for the entire trip.