According to the readme, the attack should happen when you are the leader if a great house or have been named Nereverine by one Ashlander tribe. My character meets both of those requirements but the attack has not happened after sleeping about 50 times.

Can this mod be uninstalled without messing up my save? Or can the attacks be initiated by console command?

At this point I'd probably like to just uninstall. The delayed attacks sounded like a good idea at first but I have changed my mind about that. Not having access to Mournhold is getting kind of old. But I don't want to damage the save.

I'm running version 2.1 of Avengers mod. DB_Attack_Mod.esp.

If the mod doesn't mess with the assassins themselves, then you can try manually placing the assassins

placeatpc db_assassin1 1 0 0

Killing him would get you the journal entry which allows you to progress.

or you if you are feeling adventurous, you could try my http://www.nexusmods.com/morrowind/mods/42619/?. It also doubles up as DB Attack delayer.

Turija

The readme should have a section for uninstalling so I suggest that you follow the instructions. If not then remove the mod and repair you gamesave in Wrye Mash by updating the master list to the right in the Saves tab.

If you're just looking for a very simple way to control the attacks then why not try

http://www.nexusmods.com/morrowind/mods/23923/?

It allows you start and stop them at will