I hate to seem mean spirited, but based on your post, you are woefully uninformed as to what is involved with winter camping. If you think we are all jumping on you, it is for good reason based on lessons learned, so do youself a favor and pay attention. We have a great Winter Forum where you can read through the archives and learn from others, including learning from their mistakes, mine included.

Too often we have read stories about inexperienced campers out in bad weather and too often they do not have a happy ending. Granted, car camping is a different story, but still, it should be treated seriously.

What also concerns me, is that if your list is anything like what the Scouts will be taking, they also need a complete rethink of they will be taking. Whoever is leading this trip needs to make sure that each one of them has the right clothes, footwear and accessories-gloves, hats, lights, etc.

The Winter Forum has many gear lists for winter camping and there is a Winter camping section here on the site. Mixed rain and snow is about the worse condition I can think of. I've done it. I much prefer cold, clear weather over warmer, but rainy any day.

Where to start?
What kind of shelters are you using? In bad weather, you need decent tents even if you are car camping. By decent, I don't mean expensive, but a tent that will withstand wind, rain and snow loading. Something like a Eureka or REI tent will do fine and they don't cost all that much. Whatever it is, it must be seam sealed, otherwise, you are looking at water in the tent.

Forget the straw idea, that makes no sense whatsoever. A decent tent needs at most a footprint, which can be made out of Tyvek or similar material.

Clothes-The only cotton I wear is jeans and a t-shirt while driving up to the trailhead. Once on foot, NO COTTON IN WINTER, not even underwear or socks. Even the cheapest fleece from Target is better than cotton in winter. All layers of cotton will get you is layers of soaking wet clothes that will never dry out and will not keep you warm.

Wool is fine, it will be warm, even if wet. Make sure whatever you take is real wool, not some cotton/wool blend. They are not the same thing. If it is raining or snowing, keep your jacket or poncho on. If either has a hood, use it.

You can get relatively inexpensive synthetic underwear-long johns and a long sleeve turtleneck shirt (my preference) at most sporting goods stores like Dick's or Chick's or Sports Authority. Maybe even at Target or Wal-Mart. There isn't a Wal-Mart anywhere close to me, so I don't know what they carry. I even have synthetic briefs. Mine are Jockey brand, but I imagine other companies make them. Again, nothing I have on is cotton from the skin out in winter.

Bring extra gloves, you can get Kinco brand winter gloves at hardware or feed stores. Cheap and durable according to skiers who use them. Wool or synthetic liners in light waterproof mitt shells also work well.

Stove-forget alcohol stoves in winter. I know people here use them, but for winter, especially for groups, you need a real stove, not some dinky thing made from a Coke can. Your best bet is a canister stove. There are cheap ones for about $25 or so. Not the lightest, but they work just about as well as any other, regardless of price.

Canister stoves do not work well in really cold weather, but at 30-40F, they are fine. Lower temps, I'd get a white gas stove of some sort. Don't let anyone talk you into an XGK, you don't need one and they are really expensive.

If you think you can melt enough snow for cooking and drinking with an alcohol stove, you are wildly overestimating what those stoves can do. In winter, you need to eat real food. You can't live on just Ramen or something like that. Winter sucks up a lot of calories just staying warm. Since you are car camping, I presume you mean the car is within a short distance of the campsite. If so, you could even bring canned food like stews and soups since weight won't be an issue. Otherwise, dried meats, pasta etc. to boost your protein and calorie intake.

Forget the charcoal heater. If you use it in your tent, you may get carbon monoxide poisoning. When I say "may", I really mean "very likely" so don't chance it.

Your bag sounds fine. Use the two pads. If you bag is really a 0F bag, that should be enough. My bag is a +23F bag combined with an overbag and it is good to about +15F.