check out my pic from the brockwell wood meet this weekend gone, i had an army bag, bivi, tarp, foam mat and inflateble mat, i put the mats down on the outside and have my doss bag inside my bivi bag, all i had on was my socks and boxers, and i was toasty as out......
we had snow over night and it must have been down to -3 or 4, i was fine, as above NEVER do your bivi bag up tight you WILL run out of air, i did it years ago when in Canada -30 in a snow hole, woke up going mental trying to breath...........
if you want the top closed i just fold mine over my head to help trap the heat from your breath in, works a treat and i have never had any problems with condensation either...........
you should be fine with your kit, just use loads of leaves and firn or branches/bows to insulate against the cold ground...........
hope this helps.......
chris.