I recommend that at absolute minimum, there be a bar similar to the shield gauge that appears under your HP bar when Walking Dead is active to show you how much more healing is required to cleanse the effect. Alternatively, give every healer job UI a place-able element that shows when your tanks' invulns are active, and along with a "cleanse bar" for DRK. I also think that it's fair to ask for the tooltips to be absolutely explicit in how the action works. I wrote up an example of how this could be done in the thread that I linked. The onus is on the writer, not the reader, to communicate how it works.

I don't think either of these things will make it completely user friendly to a new healer. But that's a overall problem with its design.

Let's say that a tankbuster occurs 20 seconds into a fight. You activate Living Dead at 11 seconds. Walking Dead activates at 20 seconds. Benediction is used at 29 seconds. Living Dead is back up at 311 seconds. You've mitigated 9 seconds of damage, so that's the effective duration. The ability is back up at 291 seconds after you start mitigating damage, so that's the effective recast.

Now you're on PLD. You activate Hallowed Ground at 19 seconds. Hallowed is back up at 439 seconds. You've mitigated 10 seconds of damage, so that's the effective duration. The ability is back up at 420 seconds after you start mitigating damage, so that's the effective recast.

Pre-casting Living Dead shortens your effective recast. Most invulns and other defensive cooldowns can also be precasted, though, if you don't need the full duration.