It seems obvious to me, that to allow everyone to play, enjoy and keep collecting loot/weapons during this event, the Terror modifier should have been “in addition to” any existing modifier.
That is, if it would have had an Anointed mod on it, it still would have it listed as an ‘addition to’ it, but with the Anointed modfier greyed out (and not applicable).
In addition to this, to add to the options available to the players, Maurice could provide the option to ‘distill’ (or clean, purge, etc.) the weapon to remove the Terror mod and therefore allow the item to have the Anointed mod come into play. This would then no longer allow the Terror mechanic to work with the item.
This makes items found during the event as useful as pre- or post- weapons would be, even after the event. Whether there is an option to ‘clean’ the item after the event is over would be something for the devs to decide (or allow it at a cost?).
Everyone could then continue looting, 100% in the knowledge what they find ‘could’ be useful (as much as the chance was prior to the event).