Extra care is need for stuck Monart / Vasart bottle stoppers ... because of the well known problem of easy cracking of the glass when under stress. With inkwells, because of the solidity of the stopper and shank, a break is more likely to occur in the neck of the bottle.
I suggest trying a variety of normal releasing agents, like WD40 and equivalents, followed by GENTLY tapping the neck area with a wooden spoon handle.