Nitpicking but... Living Dead isn't 17-18 seconds of Invulnerability if planned right. It's 17-18 seconds of healers being able to DPS freely if planned right. No matter how hard you plan you're only going to see 8-9 seconds of Invulnerability.

Living Dead- For 10 seconds if you die your hp won't hit zero instead you'll get walking dead debuff.
Walking Dead - For 10 seconds your HP will not hit zero if not healed for the maximum of your HP you will die.

The invulnerability is only part of Walking Dead. Living Dead is just how long you have to die to activate Walking Dead.

While healers can DPS freely during that 10 second activation time if you planned it very well(realistically I think you're only going to see about 7-8 seconds here maximum) You are not invulnerable. As in order for Walking Dead to activate you have to die.