One more suggestion with the lathe bolted down I have used a long two by four, as a lever, between two of the jaws and putting the lathe in back-gear try snapping the timber downward. I've been there even if it didn't crash. Some of the guys have bought trade-school machines that had seen all kinds of crashes who may have more ideas. Good luck
Along this same line but instead of "snapping the timber downward" get a 5 gallon bucket and put it on the end of the 2x lever that is held in the jaws. Put it at and angle and lock the backgear then put water or other heavy objects in the bucket. Soak the threads and leave it for a day or two. I don't remember where I saw this but the poster who described it said that in the middle of the night he heard a crash in the shop. It was the chuck loosening up and the bucket hitting the floor.