The NeoAir may be the cause of your cold nights. While others have said they can stay warm on theirs below freezing, others need another pad to use in conjunction with their NeoAir. I would suggest looking at a cheap ridgerest or a piece of z-lite to place under your NeoAir.

The problem with wearing one heavy layer, rather than multiple lighter layers is that you could very easily overheat and that is just as bad. You then take off the heavy layer and the cold saps your warm body moisture and leaves you cold.

I find the key is to know your own body-- put on a layer before you get cold and not after! Take off a layer before you become too warm and not after!

Synthetic clothing is fine. My favorite base layers are Capilene polyester. Down provides better warmth, is generally lighter and more compressible, but is often more expensive and is not something that is truly needed-- people have been keeping warm with fleece jackets and synthetic jackets such as primaloft work just as well, for a fraction of the cost.

As for being smelly-- meh, I only ever bring two shirts, so if I do need to wash one I have another to wear and it also serves double duty for creating additional insulation. For shorter trips, 4 days or less, I can get by with just one shirt, depending on the forecast.