can confirm killing an unstable enemy with burning stacks having final blow will not trigger the unstable perk even if beside the unstable creature. knockback cannot kill a creature if they were not the primary target but if the creature killed creature is the primary target and knockback dmg is what killed it, only the knockback damage number will be dealt to you. example: if the target has 12 hp and you have 10 base dmg and a bear mace (making 10 + 2) the number of damage the unstable will do is 2 not 10
edit: an unstable target knocked back into a wall to be killed will not inflict you with the unstable damage
edit+: knockback damage is not factored into riposte damage and knocking back a riposte enemy into an empty space/ wall will not stop you from tacking the damage and riposte cannot reflect back to you burning damage if you damage another target (example hit a popcorn while a riposte dude has burning stacks on him) but if he is the primary target the burning counts in the overall damage he is receiving meaning you take extra damage. a golem arbiter (25% riposte) with 4 burning stacks will deal a minimum 1 extra damage when hit directly.
any more questions? (yes I am vigilant at finding information)