I agree w/ Youngblood on the likely 0F range of the Downmat 9...last year at Mt Rogers the Downmat 7 got me to 20F until I started getting CBS. So...unless it's hovering around 0F anything else on the underside of the hammock is extraneous hassle and weight. Also, the Downmat 7 is just thick enough for me to not use a SPE but I'm not a broad-shouldered guy...may work for you as well.
I think the base of your system for 0-20F should be the DAM underneath with bag+NS on top of you. If you're using the bag as a quilt, I'd put it over the NS; if you're zipping up the bag, put the NS on top.
Re: not needing another VB directly against the hammock, that's true. But an undercover hung loosely may give another pocket of dead air space below the pad...I think the wind is what made my Downmat 7 give me CBS, and a Garlington Taco type undercover likely would have given me 5-7 more degrees of comfort. So, only when you start feeling cold, use the Tyvek as an undercover. And only when you start feeling cold, use the poncho inside the hammock. But I bet it will have to be near 0F before you need either of those.
Bigger issue is making sure you have something warm on your head. A sleeping bag hood rated to 30F may not be enough at 0F, no matter what your other insulation is.