if the best isn't available
Simply put, I-70 is the best, most well maintained route across the Rockies in Colorado. If conditions are so bad that they are forced to close I-70, it means that things have got so bad that world class plow crews can not keep up with the weather. If that is the case, trying to find another route across the Rockies would be a very foolish move, as there is no way that any other route will be in a safe passable condition.
Yes, you could try to go around by going up to I-80, but there is a good chance that Wyoming would also be seeing the same weather system. In any case, if that is what you run into, you're going to have to make some pretty major cuts to your plans either because you will wait out the storm in Denver, or because you'll be adding a whole lot of miles to go up into Wyoming.
I still think you are grossly underestimating the time it will take to travel up the California coast. If you don't leave SF until afternoon, you won't get anywhere near the Redwoods until hours upon hours after dark (quite possibly after midnight), completely missing out on the great scenery that is the Pacific Coast Highway.
In general, there's nothing wrong with "going" a little past dark, but you've got several legs where you're not going to get to your destination until very late at night, and thus, you're not going to have much time for sleep if your plan is to "get up at dawn."
