Great question. There are lots of folks here with sawmill and woodlot experience. I hope they join the discussion.
My guess…all the stain shows an abrupt edge on the outside of the discoloration. This would be consistent with an injury at ground level given you said the cut face of the log was 7” off the butt. Injury such as this might be caused by skidder or tractor damage, porcupine, ground fire, etc. The stain extends inward. Trees have the ability to compartmentalize decay, thus limiting the spread of decay outward to new wood. That’s why some trees are solid on the outside and hollow on the inside. Picture number 105537 has a very large stain, suggesting multiple wounds. This would be an injury that happened on multiple faces of the stem, such as ground fire, porcupine, kid with a hatchet, livestock, etc. No way to know the injury at this point. The stained wood should be solid, and the discoloration adds character. I’ll guess the stain extends 16” above the height of the top of the wound…which you probably won’t be able to detect.
No risk in sawing boards, and each board will have a story. The other conclusion is the importance of making sure that damage to stems is minimized when we or others work in the woods. As we all look forward to spring thaw (March - June), extra caution or avoidance is warranted to protect root systems and reduce bark slippage from bumps.