I can sympathize as my sinuses close up when I am around down so I have been a lifelong synthetic user. Northface makes good bags so you should be happy with yours. I have moved on to Wiggy's because the loft does hold up better Than Nf bags and I have options for sleeping with my wife and/or using the over bag when the temps really drop.

As for the base layer/no layer for sleeping,well, that is a personal and situational thing. The most important thing is to stay dry. A base layer must breath well or it will load up and you will get damp/cold. If you wear too much and sweat a lot to get rid of the excess heat then it is time to wear less. If you are dry but cold it may be time to add a layer. On the other hand, if you know you will be getting up in the coldest part of the night a warm base layer is a Godsend for comfort until you are back in the bag and in snug mode.