If your lathe is big enough, chuck on the bolt shank to face the end and center drill, then re-chuck on the head.
If not, then hold the bolt by the head in the 3-jaw, center drill for the live center. Be careful the setup may not be very stable, and the end of the bolt will not be square. Use reasonably slow speed for this operation, just in case the bolt comes out of the chuck. Use a steady rest if you have one.
Alternatively, cut the bolt head off to have a longer area to hang on to.
As far as removing the threads, just turn normally once you have a solid setup.