Just something to think about, the whole idea is that Lamb and Wolf have power over life and death, so what if when she cast Lamb's Respite, her mark could still die? Or, maybe Mounting Dread's third hit could execute them(and be the only power that could kill an enemy during Lamb's Respite - that way it requires proper skill timing and only works on a single target.)
If this worked for Marked target's, they would know they were marked before fighting and the counterplay would be to not stick around and count on Lamb's Respite to save them.
If it worked off of Mounting Dread, the counterplay would be to get out the moment you were hit with the active.
It would be a good zoning tactic for someone like an enemy Darius who wants to be in the middle of your low hp team as well.