Well that's a good post

I agree with the penality being too arsh. I have no issue with the whole "you gotta heal me" in a raid environnement, but the clunkiness of it when Benediction is unavailable is indeed bothersome at least.
First, as you pointed, it is quite annoying to not know how much heal is required
Second, even if you don't mind healing, in multiple TB scenario, you kinda throw heal out of the window because you can't just wait until the last second to heal it up. You have to start topping your DRK which get hit again thus negating the heal making your DRK still low hp once Living Dead wears off after receiving the required amount of healing.

To me the only benefit of Living Dead over Holmgang is its failproofness, you can't fail a Living Dead since you can precast it a whole 10s ahead of the TB. This is very handy for new players but it falls short very quickly.
The 10s duration would be a perk if the first issue wasn't one (for instance you keep healing and the DRK stays at 1hp until the debuff is removed, then he gets all the healing done in one go).

Finally, the way living Dead work can in itself be detrimental and cause wipe.
For instance, taking E4S TB which is a multihit TB, you have to be very careful to not overheal your DRK before the second hit land otherwise you effectively kill him. (Obviously this is mostly an issue for WHM-less party)

As I said, I have no issue with the flavor of the skill requiring healing (in a raid environnement, your concern of soloplay are quite valid), but it does feel quite weak for no / very little benefit. (Longer duration for those very rare double TB, but kinda needs a whm to work properly)

If Living Dead were to have
1- Shorter CD (at least on par with holmgang)
2- UI elemental showing how much heal is required
3- An effective 10s duration of invincibility no matter what (so that your healer don't accidentally kill you) and ideally a way to not waste the healing done as explained before

Living Dead would probably not be considered so weak and would in fact be a great alternative to Holmgang, both having their pros and cons