We always advise trying all of this at home to work the bugs out. Clearly, that's not an option, here. Assuming you will be car camping, take everything you own, and plan on spending some time working on your rig. Some of that time will probably be at 4am, but hey.
If your rectangular bag is synthetic, you can be surprisingly comfy putting it fully zipped inside the hammock under you. If it's cheap synthetic, so much the better: it won't compress much. You'll still need a pad...maybe put it inside the rectangular bag. Then use the mummy, unzipped, as a top quilt.
I'm envious...I'd kill for that forecast.