If all of the undead are going to turn back human, won't some of them die from their injuries? Kieren's wrists are only held together by stitches, Simon has had his back cut open and spine exposed also only held together by stitches, some have been shot. I feel it's going to be very interesting to watch that play out...
I don’t think they will have injuries by the time their hearts start beating. It would be really quite useless to have the dead come back to life after even just a little while, if the process didn’t involve physical healing before their hearts stopped beating, because of the way the body begins to putrefy/break down.
We put a lot of preservatives (like embalming fluid) into human bodies to keep them looking ok for funerals, but the breakdown begins from the moment of death, and even just a few hours without proper care can be enough to render organs etc useless. If the undead were to rehumanize with zero healing, it would be just to die again immediately.
We also witness Amy being full of blood when Maxine stabs her, when I’m 100% sure that if Amy was buried she was probably embalmed. We’ve seen the undead only having black liquid for “blood” in the few instances any of it is present in an injury. Amy clearly regenerated her blood through some means, and since she didn’t immediately die from the stab wound Kieren gave her in season 1, I’m guessing other parts of her healed.
My theory would be that her “symptoms” were all leading up to a phase of healing, starting with her being able to feel the rain and ending on the 12th hour of the 12th day of the 12th month- and that is when her heart started beating because the healing was done and process was complete. I think the others that rehumanize will share a similar experience, with shaking/forgetting/etc leading to a reboot of their nervous system (which is responsible for telling where pain/injury is so it can be healed), followed by a short but intense amount of healing, and ending in a restarted heart.