I usually use a tent, and a tent tends to be a couple degrees warmer inside than out. Maybe not enough for you or the differential between bag rating and actual temp, but some help none the less.
I thought you were supposed to have some space between you and the fire- enough to get out of the shelter without being over the fire. That should be enough space to keep all but the worst night-flops from turing into a firey situation. The reflector directs the heat into the shelter, not entirely how close the coals are. It that wrong?
Extra blankets, hand warmers, good under insulation, a good meal, etc. are all great suggestions- I've used them all and I think the all work. Newspapers I hear are a cheep additional solution to a pad. Not sure how much though. I like a hot cup of tea with honey and a thick hat. I wear my long underwear inside my bag, and a clean dry pair of wool sock until the bag warms up. I'm a "hot sleeper" though, at least until early morning.
Sometimes you just have to get up and start cooking- even at 4 am when you can't sleep anymore. If your shivering, it is a bad sign. get up. get warm by any means available. Have fun!!!