The stem should come out if the set screw is all the way out. Might try lubricating the inside of the pendant and then tapping the end of the stem with a small brass hammer from the inside of the case. Some of these stem setups had a round brass piece with a slot around it for the set screw to fit into. The brass piece screwed down the extra long threaded section of the stem which allowed for a height adjustment. This brass piece may have thrown up a burr which is catching on the inside of the pendant. Some setups didn’t use this brass sleeve piece and just used a stem with a large diameter shoulder which had a slot around it for the screw to fit into. It would center itself by the fit of the crown over the pendant.
Please let me know how it works out BYJWRI,