Vertical agro has been a problem for a long time now. If you can stop mobs who are not on the same level as the player from activating (and then trying to path vertically through maze like structures) it seems like a lot of your pathing budget would be freed.
Reaper snitches... if a player successfully uses stealth past a mob or group of mobs, then later finds a reaper, that reaper currently can give the player location to all the previous mobs. I would guess that a medium to large number of mobs all suddenly pathing horizontally (and maybe vertically) uses a lot of the pathing budget. Maybe don't allow the reapers to give away your position to mobs beyond a certain distance and never vertically through the floor or ceiling???
Limit mob response to invis and stealth characters and also to out or sight characters. Any time a mob does not actually see a character but does think something might be there, yes they should investigate. Send them to the "echo" ONCE. When they get to the echo, have them investigate as appropriate for their class/race etc. If they don't find the character at the first echo, then either return them to their previous activity or deactivate them. Most mobs are henchmen anyway, not continuing to search beyond the initial investigation would be the normal behavior.
In the vein of limiting the response you can have a variety of types of response (most of which would reduce pathing load). For example, you could have guards on a door that are curious. These mobs would leave their guard spot to investigate (stealth and invis could sneak around). Some mobs would be less curious and unwilling to leave their post out of duty or fear (these would frustrate steath and invis because they can't be pulled from their spot.) You could have exceptionally intelligent mobs who know the weakness of the dungeon or where the valuable treasure is that instead of going to look where they think the invis or stealth character is, run to where they think they will intercept the character (this would be preplanned/scripted path so likely less pathing load.)
The bottom line, when you talk about going to the character's echo, why do the mobs follow the character when they never actually find them? This seems like your pathing load problem. My observations say that mobs continue to go from echo to echo even without a reason to know where the next echo is. The process to stop this behavior is broken. The second problem is chain agro where mobs without line of sight are alerted.