As for the "it lasts 20 seconds!!"

It doesn't.
Living Dead lasts for 10s, and Walking Dead lasts up to 10s. You can have LD shift into WD at any point in those 10 seconds, and WD can be cleansed almost whenever in those 10 seconds. Holmgang's one deficit to it's duration is more felt in multi-enemy situations if used on a target that dies, whereby you lose Holm. Learned that the hard way. Living Dead doesn't do jack-all but allow WD to trigger. "But it's a sign to your healers to not heal you!" So? I've been not-healed by my healers enough to say LD isn't any more an incentive than anything else for me to just be left sitting with a regen and TBN on me.

---

spitballing ideas here: tying back to how people have so many things against LD, and DRK just keeps getting skills pruned.
What would be the downside if Walking Dead was adjusted so that any damage taken while under this effect is instead restored as HP to the DRK?

People keep saying "just add in a healing increase to walking dead! like convalescence!" Why settle for just that? It'd still be in line with DRK's lore of persevering through adversity. If anything for the sake of "balance" just make it so that you still die if you're not fully healed after WD's duration, or just inflict a penalty to WD that prevents you from being healed by anything but it's own effect while you're under the effect of WD.