I'm with
@CWLONGSHOT on this one. All that keeps the mandrel from falling through is a very tiny lip on the top of the mandrel, so the hole the mandrel goes through is pretty close to the diameter of the mandrel - maybe enough so that the hole will stop the case mouth as you tap the mandrel "up" through the hole. Risky for the decapper pin, but well-centered, light taps, especially after the Kroil has been sinking in could possibly do it.
If the case neck moves some, but hangs up, stop, push the mandrel back in and file a weak spot on one side of the neck. If you get close enough that there's just a thin bit of brass on one side, it can loosen or lessen its grip, or allow it to be peeled off. Had to get steel bushings off several bolts on a Jeep suspension this way this spring - but I used a grinder. A file would be more appropriate/safer for the mandrel in this case.