In my mod, an NPC summons a ghost during a scene, who is supposed to use a force-greet package to speak to the player. As far as I can tell, it should work- but it doesn't.

All I can come up with, is that somehow the summoning spell effect wipes out the summoned creatures AI packages and replaces them with their "follow player" package. Is this true, and if so, what can I do to make my ghost be summoned, but still use his fore-greet package?

This is a known problem. The best you can do is use placeatme command instead to create it and disable and delete when the effect fades or use an essential character and use moveto to teleport it where it's needed and use it again when the effect needs to expire or when it enters bleedout.