On further testing conserning aggro, it looks like mirage clones has their own aggro bar, which gets transferred to the spawner the moment the clones despawn. Here your see my companion Paranoid Delusion (looks like a 4th mirage clone) having aggro and getting hit by Zariel. In the next video frame, my mirage clones…
On further testing it seems like Undead Slayer Overload does not work on any undead mob. So "Palhavorithyn and Valindra" is probably just marked as undead and not as dragon, that would explain the results.