Some points.

As long as Living Dead has a penalty attached to it and Holmgang does not, Holmgang should be on a longer recast. Superbulli and Hallowed follow that rule for a reason.

Second, if you want survival to be in the hands of another player, the ability has to be as simple as possible so that everyone involved instantly understands what is going on. If you take SB's penalty as an example, it's very visually obvious (i.e. suddenly, your tank's HP is 1). On LD, there's a debuff that pops up, and if you hover over it, there's a wall of text that you can try to decipher while your doom counter ticks down. And even experienced players don't actually know how much remaining healing they need to pump into you. It's either bene, or slam in those heals until it disappears. Really bad design.

If you want to keep things roughly the way that they are, everything needs to be made more obvious. Place a red counter that starts counting down from 10 to 0 over the DRK job icon in the party frames. Place a black bar next to the DRK's HP bar that disappears as you provide more healing, letting you see how much more healing is required to meet the check. Also, make it so that meeting the check doesn't remove your invulnerability. There's nothing worse than dying to a follow-up tankbuster because your healer cleansed the effect too quickly. And perhaps either add a convalescence effect to the ability or reduce the %HP required to cleanse. We're getting into the 120k healing range now, and it keeps going up.

An alternative solution is just to take other players out of it, like all of the other invulns. For the next 10 seconds, your HP can't go below 1. When you hit 1 HP, it changes to: You can't take damage from most attacks, and you can't be healed by other players. All your attacks grant you additional lifesteal. Now it's all in your hands. Just you and your opponent. The flipside is that if there's nothing to beat up, you probably will die. But it's more interesting when it's something under your control than another player.

Any invuln design that gives you a dps boost is going to be exploited, so it's best to keep that out of this.