If it were me, I'd opt for the warm clothing and not rely on the bivvy. I haven't found the emergency blankets to be a viable option. Fleece or wool top/bottom along with wool socks, used only for sleeping, would keep you warmer AND more comfortable. Reflectix under your pad will be a big help also. Mine is sized to cover from waist to shoulders, I've found that if my core stays warm I sleep well.
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