They do have a root cellar, but it is built into a hillside on the other side of the crick, just below the big pen, not under the cabin. A back door sure would have come in handy, and they were just about to put one in.
Lucas was going to do some explaining, and we can all suspect it would have been illuminating, but now it is up in the air whether he will ever get that chance again.
Blood loss is a life taker, and it would seem Luca has lost quite a bit. I think we see in this chapter just how resourceful Jubal is, though. He did some damn quick thinking, and the fact is he is completely in love with Lucas. This has got to be killing him... staying rational when all he wants to do is scream and cry.