I'm pretty sure (judging from various chatroom conversations & general forum reading) that once a character is made immortal there will be no way to remove that immortality. Whether that is due to the code or just the gameplay, I am not sure.
Invisibility allows you to make a character invincible (not able to be targeted by your opponent) but can be removed either 1) through Flash of Light or 2) through that character using an ability.
So Quintessence is ultimately totally different from Invisibility. See my strategy post re: invisibility, werewolves, blessings and chaos power.