Very interesting questions. Thought I'd chime in with my thoughts on shelters:
I have concluded that a highly breathable wp/b bivy, such as the eVent bivies you sell, combined with a poncho tarp (I personally use a Gatewood Cape), provides an excellent combination of UL comfort and disaster preparedness.
UL philosophy assumes you to have an ability to descend to lower, protected areas. But I've never felt comfortable when crossing a high pass on a solo trip knowing that, if I were to be seriously injured, I would not be adequately prepared to spend a few days up there.
Even though a wp/b bivy and tarp are technically both complete shelters, I think of them as complimentary rather than redundant, and they are both multi-use items.
I'm not sure how helpful a long zipper would be on a bivy for survival situations. You'd still have to be able to reach down and zip it up, so you might as well be reaching down to get it over your legs. but a shorter zip would certainly help.
One of the reasons I like a quilt is the ease of getting it over my legs in an emergency situation. I combine a 20 degree down quilt with a cocoon pullover and pants. It's overkill for most situations, but I feel will keep me safe in pretty adverse 3-season conditions and is still lighter than a 20 degree synthetic quilt and jacket.
Actually, I would be very interested in your thoughts on the following matter: The reason I have selected a Gatewood Cape, aside from its poncho feature, is the full protection from all sides, which is helpful above treeline. Of course, I don't need this protection with a wp/b bivy. The real reason is that I believe I am creating a quasi-double walled shelter with this setup.
Even though I am reducing ventilation somewhat, I am creating a temperature gradient from the outside that allows the eVent to breathe properly. My experience with eVent is that I can keep myself quite dry inside so long as the outside surface is not too cold or damp from dew. The more protected Gatewood keeps the temperature inside warmer, thereby keeping the inside of the bivy drier, even if the ambient humidity is a bit higher in the Gatewood vs. an open tarp. Perhaps the same could apply to your shaped tarps, including your new Trailstar?
Of course in warmer weather below treeline I will not use the bivy, and I will keep the gatewood awning partially or fully open.
But this is based less on experience and more on extrapolations from various articles on this website. I'd be interested to hear what our expert shelter manufacturers on this site think about this.