There is no dialogue in the dialogue menu for NPC "PresidentEden", and there is no dialogue menu at all for talking activator "MQ09PresidentEden". A text search for "This has to end" produces dialogue topic "MQ09EdenSelfDestructSPEECH", which implies that there is an NPC which does have this dialogue option, but the search feature doesn't tell me where to find this to edit it. Can anybody help me find it, or know where it is?

Oh my god! I found what I was looking for, completely by accident! I idly decided to "Edit player dialogue", an option under "Character", then looked for MQ09, not expecting it to work, and was then able to access all of Eden's dialogue from there! It turns out all of his dialogue is under the quest dialogue for MQ09. So yeah. Problem solved, I guess.