I'm not sure why you suggested using a 0° top quilt, but if you used a 20° top quilt, you would save approximately 10 ounces compared to the WM sleeping bag. I've used my 20° top quilt down to the mid teens without supplementing with clothes. It's darn warm. You could get the top quilt lighter by using M50 fabrics, but it's pricey.
Gatorade bottles are lighter than Kleen Kanteens but they are not suitable for very hot water.
Did someone mention the Darien UL Hammock yet? That will save significant weight compared to a double layer Blackbird.