Does being "prepared" involve settings Atoz has to switch on that "transforms" the person when they pass through, or is it something done to you just before you enter?
That's something the script didn't make clear. ... it seems you can travel through time for a short period without preparation. But once prepared, you can't make the trip at all without dying as soon as you reach the other side.
Over the years I sometimes wondered if the death aspect was just a lie told by the authorities, to prevent unauthorized use of the Atavachron portals by exiles trying to return and/or fugitives trying to escape. It never made sense to me that time-travel on Sarpeidon would work differently than in the rest of the galaxy. Nobody else ever had to be "prepared" in advance, even those like Tasha Yar who remained in a past timeline, and the stuff about compatible cell structures and brain patterns feels like hand-wavy nonsense in the Trek universe.
There's also a whole speculative rabbit trail about how the authorities gained the massive amount of intel about the past that would be necessary to use this device without wrecking their own timeline. How they learned which areas of the ice age environment were survivable, how the Atavachron testers safely came and went, and so on.
But in thinking all this through again now, I had another idea that could accommodate the story details. Bear with me while I finalize my thoughts via keyboard.
The Atavachron system seems to comprise four main components: a disc-based Selector, a Preparer apparatus (unseen, IIRC), a Portal, and a Computer to tie it all together. (If the Curse of Fatal Death part is all just a big lie, then the Preparer doesn't do anything except reinforce the idea that you'd better not try to come back.)
Now let's suppose that
two of these components actually modify travelers' bodies, in some chemical, anatomical, or genetic manner. I'll use medical terminology as an analogy.
- The Preparer apparatus simply injects a tag or marker, the sole purpose of which is to inform the Portal of the traveler's status (approved/prepared vs. unapproved/not prepared).
- The Portal scans each traveler and optionally injects a countdown timer; a poison.
- If the traveler lacks the prep marker, poison is
- injected if the traveler is departing, or
- removed/antidoted if the traveler is returning.
- If the traveler bears the prep marker, poison is injected only if the traveler is returning.
"Preparation" is basically just getting a bodily passport; the Portal does all the real work.
- Kirk & co. had no passports, so they got the poison upon departure and would have died later that day, had they not returned and been auto-antidoted by the Portal.
- Zarabeth & the magistrate got their passports, thus received no poison, and will get auto-poisoned only if they return.
In this view, the government still is selling a lie, but not a lie about
whether you die; a different lie about
why you die, with brain-pattern and cell-structure red herrings, when in reality it's an unnecessary "feature" purposely introduced to the mechanism. The lie is covering the fact that really nobody would have had to die either way, but they added this complication to their time-travel device in order to maintain control and disincentivize fugitives and repatriates.
Under this headcanon, if McCoy had had time to figure it all out, and remove Zarabeth's marker, then Spock could have had another tragic and uncomfortable "the only reason we can't be together is that I got my marbles back" moment, this time with with Zarabeth. Maybe then in some novel she could've gone all
Thelma & Louise with Leila Kalomi.