conceptually both banish and instakill achieve the same result - they instantly take away an enemy off the map. but mechanically are they the same?

theres pets whos abilities allow them to banish mobs, while others can instakill. whats the fundamental difference between the two if at all?

if theres no difference between them, can the two stack? like banish chance + instakill chance = x% total vanish chance - does this work or am i completely off base here? does banish stack with banish or instakil with instakill?

lastly, what does "lesser enemies" even mean? is it like all enemies who aren't either named mobs, minibosses, bosses or players? or something like enemies below a certain health threshold?

tia for indulging me and helping answer them.