The mine props cut from the family farm were just that, lengths cut specific to fit the mine ceiling height, then shipped on a truck usually stacked cross-way. Ceiling timber was longer and usually shipped as a single load. There wasn't a lot of ceiling timber cut. The lumber cut from the farm was hardwood, usually oak. Short lengths, to match the coal seam, cheap lumber, but strong. Usually much smaller trees, than cut for prime lumber. There wasn't a lot of saw mill work for this lumber. IMO. The first picture is very interesting. Second picture shows the mine props, as I remember them leaving the farm, complete with the bark. A mine ceiling height of 5 ft would have been big in Western Pennsylvania. 3 ft. to 4 ft. and, smaller more the norm. My guess is most miners worked on their knees.
Pictured is probably a room. Most shafts would be supported by the coal not mined. Rooms were larger, more coal was taken, requiring the props. Long wall mining takes all the coal, allowing the ceiling to collapse. There have been attempts to long wall mine coal left from previous shaft and room mining with limited success.
I could be wrong and often.
That being said, I really like the sawmill idea.
Mike CT