Jun, your questions lead me to believe you don't have much experience, so I will try and give you as much good information as I can.
Sleeping bag ratings are based on one thing: Survival. Comfort is very different. At 20ºF in a 20ºF bag, with no clothes on, you will be very cold and you may not be able to sleep. However, you will be alive- you will not get hypothermia unless your bag gets wet or there's a really high wind.
Your 25º bag is inadequate for -14ºF temperatures. You will have to buy a second sleeping bag around 20º to be comfortable (and alive) in that temperature. You will need to use both bags at once, one inside another. That is going to make your pack heavy, but carrying light weight in -14ºF temperatures means spending a LOT of money on very good materials. A Sleeping bag made for those conditions that weighs little will be over $400.
Down and Synthetic are different, but the warmth is relative to how much there is. Synthetic is warmer than down when it is wet, but it usually weighs a little bit more. The difference is slight. The only way to tell if your down liner will be enough is to know how thick it is. Look for a weight on the tag, or a fill count (my down jacket is 650 Fill, most sleeping bags are 800-900 fill).
If you use a good down liner, your 25º bag, a fleece blanket, two pads, and a liner, you will most likely live. I would try to read some articles on winter camping so you know what to expect. This trip may not be easy to do with your current gear, but it is likely possible.
If you can spend some money, you can get winter gear that weighs little and will keep you warm> But, you have to spend the money... Do not go into -14ºF temperatures without a heavily insulated jacket and pants, plus waterproof boots, thick gloves, a thick hat, and many layers of clothing.
DO NOT risk your life. Keep asking questions!