Most wells are in limestone. The pipe and the pitless will all break if things freeze. If the pitless breaks just right, you could lose the pump down the well.
Can you add dirt over the pipe and pitless? If not maybe insulation and dirt.
Or add a valve below the pitless, and the frost line, that can be opened but, the water in the line from the pitless to the cabin would have to be drained also.
Thanks for your input, everyone else's also. There is a slope from the well to the cabin, so the system would self drain if I open a valve at the lowest point, so the line to the tank and cabin should be OK. On the old well, using a jet pump, after removing and draining the pump, we never drained the black plastic lines from the old well to the cabin. They always froze since the limestone comes right to the surface, but they never broke or leaked, has been that way for 50 years. My guess is that the black tubing must expand enough when frozen not to break. I think I will proceed with my original plan, to put a 'T' in the drop line to the submersible pump and then run a parallel line to the top of the casing and cap that line. Then at the end of the season, will shut off power to the pump, drain the tank and lines and remove the wellcap and uncap that parallel line and using a hand bilge pump or a small electrical pump, suck water out of that line which is connected to the main drop line. I would put the connector 'T' to the main drop line down about 24 inches. Once that water is removed from the lines it should be below the freeze line inside the casing, so there should be no water near the pitless adapter inside the well, or outside the well. Since the limestone comes right to the surface, digging down a couple feet to install the adapter is not possible, and covering up the well with dirt and insullation would create quite a mound and because of its location, not too practical either. Anyway, I really appreciate your input, many thanks. Other ideas or suggestions always welcome. All the best....ken