Often, when brute force fails, a bolt of lightening will do the trick. I'm saying, speed over power--a quick, sharp, rap on the tommy bar inserted into the thread adapter may win over slow, powerful force. It may take dozens of such raps before you get results. If you are at all familiar with an impact wrench that pro mechanics use, they rely on the same principle.
If that doesn't work, then leverage combined with brute force is the answer. All along, I've assumed that the insert has a hole in it for a tommy bar--my old Vicmarc does, but I'm not sure of the newer ones. I think you're best off with the chuck mounted on the lathe, with a rod in that hole that's long enough to brace against the lathe bed and hopefully not do any cosmetic damage to the bed. Duct tape it to the chuck to keep it in place while you're wrenching on the chuck.
Then, take a 1 1/2"-2" square stick of hardwood, at least 18" long, and use that as the other lever between the chuck jaws, perhaps clamping the jaws down hard on the end of the stick. You risk damaging the chuck if not too careful, but using a wood lever instead of a steel one offers a reasonable safety factor.
Good luck. Remember, it's mind over matter--that thing's gotta come out one way or another!
