That Reddit person's idea is overall good but the DRK cannot be healed during invuln so it will still be confusing for healers (especially for casual players) since they need to pay to Walking Dead buff. Press heal 1 second before the buff wears off and you're wasting your juicy heal. Also if the DRK faces a situation where they need to pop the invuln at 5-10% HP. What happens after the invuln goes off can kill the DRK, basically it's like a way inferior version of Hallowed Ground.
My suggestion is to simply make DRK invuln based on a shield, aren't they supposed to be the barrier tank anyway? basically when activated, if they get hit by a lethal damage the DRK wouldn't die, it'll stay at 1 HP and gain a huge shield (preferably with another color than your usual yellow/orange to make it easier to spot), by this time the invuln buff wears off and they will survive by relying on the shield (a separate buff). Then it will be up to the healer to top the HP back when the shield is up. If the shield wears off and no healing was received, then the DRK will die from tickle damage. It's much easier to handle since healers just need to pay attention to "does this DRK have a stupidly huge barrier on their HP bar? if so I spam heal".
To maximize it, the timing of it's usage will different than the other invulns though. But at least this will be controlled by the DRK and not the healers (wherein lies the confusion with current Living Dead, another person having to pay attention to your debuff caused by yourself). Right now when there's a 2x TB upcoming, in most scenarios tanks just use their invuln and forget about the TB since they cannot go below 1 HP. But with this shield invuln, there is no guarantee that the second hit won't break the shield (and if it breaks it will be the healers heal that determines if the DRK will leave or not).
So to gain the most out of it, it'll be something like : 1st hit = eat it -> the DRK is at low HP -> pop the invuln -> 2nd hit -> DRK goes to 1 HP and gains a huge shield -> healers top DRK.
It's not the best idea but since they already have barriers around, why not improve on it if what they're after is something unique. No need to add an overly complicated system. Also barriers are fun. Looking at a huge barrier is satisfying and gives you a feeling of comfort than seeing a debuff that will kill someone if you don't heal them enough.
I also feel like the invuln should be based around the average player's skills. So the healing part needs to be easy. Why? because imagine you're the rank 1 DRK. Then you pop your invuln in a dungeon but because how it works is too big brain, your healer struggles, makes a mistake and as a result you're dead. Not your fault, not (fully) your healer's fault, but you're dead. It's just unfair to everybody. Invuln should save you from a wipe and not causing it.