No, it stops at the moment of death. You can't measure when a zygote won't implant until it is dead (cellular function has ceased). Zygotes are 'un-implanted' before they would implant (and are therefore alive) as well as after they are dead and you can't know when that happens. Declaring an un-implanted zygote as dead leaves the door open for killing pre-implanted zygotes by allowing them to be defined as dead simply because they are 'un-implanted'. That's the wrong path to be on, IMO.
"I am also pointing out that your heart can stop beating and you be declared dead, but that the cells in some of your organs are still viable and living and can be transplanted to others."
Unfortunately not applicable to a zygote. It's either alive or it's dead and that is based on the internal cellular function within the zygote, not on whether it has implanted or not. You already admitted that a pre-implanted zygote is alive. Well, a pre-implanted zygote is 'un-implanted' and would be considered dead using your definition.
You certainly seem determined to sacrifice zygotes that you define as dead. What's up with that?
Not so. A zygote is on a timetable. It has a window in which to implant or it won’t survive. Once past that prime turf, it’s not going to happen. That explains the many fertilized eggs that are sloughed off