In my (likely unpopular opinion), just make the invuln a role ability and make it the same for all jobs - whether it be HG, Holmgang or Living Dead. Of course, HG with a lower cooldown would be king, but that's only if the cooldown is lower. As the OP said, most other tanks can heal to full (or not need to heal at all due to the nature of HG). When SE addressed Living Dead, they should have addressed Superbolide.