Thank you for the good information you share, and your good questions.
My hens got up once or twice a day. Once when I picked them up. The other time on their own. The longest they ever stayed off is 30 minutes. When I pick them up and take them off the nest I just pick them up and move them over. I don't carry them off to another part of the yard or take them out if the coop. If they want to go right back they do, but usually they go relieve themselves and drink some water/eat some food. This has been important in my situation. I have had a few occasions where ants invaded the coop.
Neither of my broody hens will poop in the nest. When they are off the nest they get a drink of water and relieve themselves, and then go back.
I hear and understand that your hens go into lockdown, but my hens don't do that. I mark on the calendar when I put the fertile eggs in the nest, and then three weeks later I wait to hear peeping. You can hear them peeping long before they hatch. Both hens got up off the nest while babies were peeping. They were not gone long, but they drank water, ate, and pooped.
It may be true that the hatch rate would have been better if the hens did not get up, but there were several factors that contributed to the low hatch rate. The first batch of eggs had three fertile out of nine. Of the three fertile, one was stepped on by another hen and one stopped developing. The next batch had three fertile out of nine - one of the seven Bielefelder eggs and both of the Niederrheiner eggs. And all three hatched. If I had to speculate I would say that the Bielefelder roo was on the particular side, and prefers one lady over the others.