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Outside of the immunity to Charm it was still a decent use of an action/ready action.
I think you could rule either way on if the zombies/lich would jump off a cliff after the gem, since the spell mentions that they specifically stare at and approach you (not the gem).
So you better jump off and cast feather fall.
Casting a spell and holding it uses visible components the hobgoblin could react to.
Incite greed also explicitly says that the creature avoids obvious harm while approaching you and does nothing beyond approaching you. If the would always run after the gem (forsaking personal safety to do so), this would be noted in the description.
Casting a spell and holding it uses visible components the hobgoblin could react to.
I'd think it'd take at least an arcana check to know that the spell you're holding is Jump. So yeah, he could react, but would he necessarily be able to react in time to stop you from casting the spell? Not to mention, if he reacts just barely not in time and jumps haphazardly, one could argue the Wizard jumped farther and therefore wins.
If he doesn't know what spell you're casting, that means that he's even more likely to assume that you are trying to cast an harmful spell, making him attack you. And casting a spell takes an action, basically a third of your turn if you want, so the hobgoblin has at least 2 seconds to react, if not more. And thats plenty of time to stop himself from jumping.