According to "The Chicken Health Handbook" by Gail Damerow, most varieties of lice attack the growing feathers rather than suck blood directly from the chicken. Perhaps lack of a set of healthy feathers combined with the onset of cold weather is weakening your chickens and making them more succeptable to disease.
I'd sure switch them to unmedicated chick feed for the additional protein content as they recover. Adding electrolytes/vitamins to their wather is good too. Continue treating whatever illness(es) pop up. Free range is good for healty chickens but compromised chickens may need more protection from the elements until they strengthen.
Any possibility of bringing some of that yummy oat grass to them or is it too labor intensive for the number of chickens? Are they roosting in a draft-free location? Can you add a couple of heat lamps during cold nights? I use a "Thermocube" which is a double outlet plug into which you plug a heat lamp or other heat source. At 34 degrees F the microswitch turns on the heat lamp.
As your flock number decreases don't replace them. When you have a manageable, healthy stock, breed them and strengthen your own strain. Just be careful of inbreeding.
I envy you for the greenery growing this time of year and that you have good dirt to make it grow...
Good luck with your homeschool business. Learning curves are good and steep. Persistence and recovering after a setback is a good, practical life lesson.
....well they need it more than I.
