The one major plot flaw that glared at me after thinking about the story is, once the spy reported what he found, why would the folks "up front" not send him or others further back -- to rescue the political prisoners sooner -- before many of them snapped?
That seemed to entirely make sense based on their description of time travel.
To reach a particular point in time, there needs to be an anchoring station at both ends of the time tunnel. If the other end is not anchored, then things passing through will end up more or less in the right place and time, but with some random swings.
In this case, once both ends were established, then you can think of the two time-periods moving in step with each other. If one year passes at my end, one year also passes at the other end. So the time tunnel doesn't lead to a fixed destination time (which would be impractical anyway because everything you sent through would arrive simultaneously) but to a time which is at a fixed interval from this end's time.
So it took some time for the political climate to change. During that duration, as much time has passed at Hawksbill. At that point, the new regime decides to send somebody. They have three choices:
1. Build a new time tunnel beginning, which may be prohibitively expensive, and just send a person through without the end established. I would not want to be that guy because they could end up centuries off target too early or too late, might end up appearing a mile in the air and falling to their death, etc. When they built Hawksbill they sent all the parts through this way, knowing that some would be lost, but sending people that way would be too cavalier.
2. Build a new time tunnel beginning AND the destination at the other end, but aim for an earlier destination. If the timeline is fixed, then you already know that you failed at this because there is no other station. If the timeline is not fixed, then at the very least there would be very unpredictable results by trying to build the two stations instead of just the one.
3. Send someone through the established time tunnel to the same station. This requires no extra building expense, and although it is dangerous in that they don't know how the people at the Station will react, at least they're not just chucking a person through to a random variation of the desired destination to a probable death.
So, I don't think there would be any other reasonable way to approach it.