I think Desperado has a point.
Being in the tree stand for 12 hours just gets you cold to start with.
Being with buddies helps.
Having plan B(back up) might save your life.

Plans C, D, E, F, G and Z might not hurt either.

In other words don't go putting yourself into a survival situation just to practice survival skills.

Try this somewhere that you can go get warm if it does not work the first time.

I once learned in the army that there was no need to practice being uncomfortable. Uncle Sam will make you uncomfortable soon enough.

To this end, my family "trains" next to or near a 40' 5th wheel that makes for nice shelter if plan A fails miserably. One night spent awake because the shivering would not let me sleep taught me two things: Build a better shelter; Always have plan B.

Ok,

Backups will be my bag and bivy.
One central fire burning all night long, with a wind break and reflector.
Hot water next to the fire
My car 10 min walk, 5 min run away.

this is a good time for a "lets try it out" story..back in the 70's i knew some guys who wanted to try Real Winter Camping..they went up to the canoe park in Northern Minnesota and skied back into one of the lakes to camp. it went down to 30 below that nite..none of the gear kept them warm even with doubled up sleeping bags..they were up most of the nite sitting around a huge fire. next AM back at the car they had to put pans of hot coals under the oil pan to start the car..ice fisherman do camp out up there but they take big tents and wood stoves..
it's 6 below right now..i don't have any plans to go anywhere.
think desert in reverse---

OK, you seem determined to do this, so...

Leave the door of your car unlocked.
Your hands might get cold enough that your find it really hard to turn a key in a lock. I have seen people that near to frozen and have been there once myself.
(besides, if anybody shows up intending to break into your car they will just bust a window out whether it is locked up or not)

If you start shivering end the experiment right then, that is the first stage of freezing to death.

If your weather conditions include 6 inch of snow then constructing a Quinzee snow shelter might be an idea.

It might be worthwhile watching Ray Mears Extreme Survival - Arctic Episode for some tips.

All is well.

The day started out 9 am in single digits, warmed up to mid 20s. Then at night it dropped to about 15. We did 3 lean 2s with a fire in the middle. No bag, no bivy. Just about 6 inches of marsh grass, and a contractors bag.

Our site was out of the wind which raged about 15-20 mph all day.

Got cold at night, when the fire died down, especially being on the ground. Just got up, added wood to the fire, drank some hot water, then repacked the tin can with snow and tried to sleep again.

Did you get much sleep? Take turns tending the fire?

I am very glad to hear it went well.
I am quite glad you did not have to run to the car or need help.
Very, very glad you had alternatives in case it did not go well.

Not quite so happy you didn't sleep well.

One night of bad sleep is no big deal, but more than two and you are likely to be sleep deprived and not functioning nearly as well as you should be.


What would make it better?

I don't know what your fire was like.
One of the tricks I learned years ago was to put some fairly big pieces of green wood in the fire (shepherd stove) and let it become extra fuel as it dried out.
It made for a more smokey fire and sooted/creosoted the stove pipes but it gave a more even heat through the night.

Some people I know like to build their fire on top of a couple of green logs.
As the fire burns it cooks the logs dry and starts to burn in the crack between them.
It might work for you guys too if you are not already doing that, worth a try anyhow.

When you say swamp grass do you mean the round hollow pith filled stems like cat tails, or the sedges?
(Rushes are the round ones, the sedges have edges.)
These are great insulators and if you have some cord a lot of them can be beaten into a sword mat to use as a blanket fairly quickly.

A trick an old friend had when hiking in the fall was to carry a flat sown bag, It was just two old bedsheets stitched up the sides and bottom. He would stuff it full of grasses and leaves then use it as a cover.
He tended to choose spruce trees with fairly low branches to sleep under and would make quite a pile of dead needles to lay his tarp on before bedding down on it.
I tried this a few times and it works fairly well, maybe not as nice as a lean to and fire, but doable.
(more modern materials like light nylon instead of cotton bed sheets should only make it work better.)

Again, If you try this make sure you have a safe out from it if it does not go well the first try.

And I was quite worried from your earlier description that you would only have wet soggy leaves to work with.

Not much sleep I would say 2-3 hours max. Whoever was up tended the fire.