So we've got a few pieces of the puzzle in place, for a simple what-to-do guide:
In a bit more detail, it seems as though any mob which shouldn't be in these areas (invaders) count, and are found throughout the area in events. Portals are involved, though don't necessarily count towards the counter.
There's been a few folk complaining their counter isn't changing, so some part of this may be slightly bugged.
When it comes to Lost and Found, we're hunting for a few items across these two zones (Diessa and Wayfarer). Some people reported looting these items from mobs during the events mentioned above, I cannot confirm or deny this, though I personally haven't had anything of the sort.
The two items I have found as of this writing are the Goblet and Wooden Soldier, one from Wayfarer (seen in the video) and the other from Diessa, found north of Oldgate Waypoint (west of the zone). When i relogged to attempt the good old overflow trick, the areas disappeared. I guess this either means I can't get that item at all or the location of it actually moved. Either way they may be a one-time only loot, and 6 are scattered across the zones.
This is what I've found out personally and conferring with a few friends, anyone have anything to add?