A few suggestions to, IMO, improve the event (some from myself, some from Eric M on youtube):

-Just let the event run until HoT releases, it makes sense lore-wise, Mordremoth took out the pact fleet and is growing in power, now he's trying to invade other areas again.

-Up it to 30 stacks for the max reward, make a gold event reward worth 3 stacks, silver 2 stacks, bronze 1 stack. Doing 1 event fully and getting gold is worth as much as waypointing around 3 times tagging stuff for bronze.

-Add an extra bloom that spawns after each event that you can harvest which would get you an extra 5-20 blooms per hour.

-Let people leave the map and retain progress. I don't really see an exploit from it, and it would mean once they cap theirs (which as you suggested would be relatively easier), they can leave, letting others come in, and letting the capped players get back to other stuff (logging off however should still cancel it, so you cant get it on multiple characters).

-Also, maybe would be nice to have constant attacks, and no break in between (one map up at a time).

-Add an additional unique currency for each specific event type (catapults, shields, and bombs) that can be traded directly for either each of the different Mysterious Keys or a bag with a common/fine crafting material (T5 or rarely T6) of respective value based on the complexity of each event. (Catapult is hardest, so best loot, but only slightly; bombs next; shields last). I don't want to encourage people going around searching for only 1 type; it shouldn't be hugely powerful, but enough to encourage diversity and the desire to do more than just the easiest ones.