MOST of the time in that area, your 15 degree bag will do the trick - you'll have clothes you can wear to bed if in doubt. Bivy will help a little.
The CCF pad - unless it's a winter-specific pad, like a ridgerest deluxe, you'll probably want 2 pads stacked. The tarp - fine if it doesn't snow and isn't windy, but I would expect it to snow and be windy, so you want something that will block out the wind and blowing snow. Many folks use pyramid style shelters(Megamid,Megalight, Betamid & lite, MLD duomid and supermid, OWARE pyramid, Golite hex, etc) in the Sierra in winter with good success.
Snow trench and tarp can work, if the snow consistency is amenable, and if you have a way to close off the end of the trench to keep out the blowing snow - another small tarp could do that if you rig it right.