1. The BD Betalight is a massive shelter, easily capable of sleeping two people (maybe three with poles angled). I could sit up, move around, and use the area between the doorway and first upright pole as an effective vestibule. As expected, there is no ventilation other than the door and any gaps you leave at the edges and condensation condenses into ice on the interior walls.
2. Sleeping with your boots (mukluks in my case) inside your sleeping bag, stashed in my sleeping bag's compression sack, is an excellent way to keep them from freezing overnight. Unfortunately, it also melts any snow and ice on the boots, which the boots them absorb. Pick your poison: have frozen boots or warm supple boots in the morning (which may eventually freeze-up). I chose the latter in my continual winter education.
3. Patagonia's R1 fabric makes great base-layer bottoms (link to updated R1 bottoms). Just like the venerable R1 Hoody, it is tight to the body, warm when sitting and breathable during high-exertion. Only PowerStretch fleece could possibly be better for the function.
4. Tyvek is great for protecting a tent or bivy bottom from abrasion. But it is not waterproof, and should not be used as a ground cloth when one's sleeping pads may not be wide or long enough to completely protect one's sleeping bag.