1. Standard is what you would normally use. "Smart" was the beta designation for what is now Google Adwords Remarketing, a related but separate product.
2. Yes, the Conversion Label is that static value. Some sites have multiple Adwords Conversions pixels and will use different ones in different situations.
3. Yes, as long as the subtotal (_subtotal) has a value on the page you want to fire, that's what will be used. If it doesn't, you'll want to use a static value.
We are in the process of updating this tag to make things more intuitive.
Hope this helps!