I have to second glenn here and say that I think layering bags isn't a good option. While I think they would definitely add insulation I think the compression of the loft you would get by layering would reduce their insulation but also, my experience has been that the relationship of insulation to temperature probably isn't linear i.e. if you want to get a zero degree bag, you can't add two 30 degree bags together, then to get a -30 deg bag you just need one more 30 degree bag! Some braniac engineer out there can explain it better I'm sure.
also, sleeping in a bivy in these conditions is rough (as you found out!) a tent will give you a pocket of air to warm up a bit more, if you can get it set up.
Have slept in quinzees a few times and they definitely beat a tent or bivy if you have the time to make them (but solo I probably wouldn't try). Also if you're climbing to 13,000 feet though, you will be plumb exhausted trying to make one.
I have not been impressed with the bag liners, they don't do much for me. a hot water bottle in your bag helps a bunch though.
My best suggestion would be to try out some combinations in more tame environments (camping closer to your car and/or with other people) before you head out to the high country. A warm snuggle bunny is always a good option too, and free!
good luck and stay warm!
A warm snuggle bunny IS always a good option, but they tend to be the most expensive/maintenance intensive pieces of gear.