I don't think much of the Chernobyl-series. I only watched the first 20 minutes, I have to admit (I'm phobic of radiation, so I sadly had to stop), but read a lot about it. I really wanted to see, how they put it to film and which differences I'd find to what I know about the accident (and I know a lot about that, I'd say).
The radiation burns they show, appear much too fast. And that's only one thing I remember about the false depiction of things. There were much more, I do remember that.
You are right, though, that there was a lot of secrecy involved. Some information that was not conveyed was not communicated because of lack of knowledge at the time being. Other information was consciously kept secret.
For example did the firefighters not know, that there was radiation involved. They thought it was just something burning. But then it was not known to the people who called them, that the hull of the reactor was breached.
Another example is the fact, that at first, they did not know, how high the radiation really was. But they did know, that it exceeded their geiger counters. Nevertheless they only communicated the numbers measured, not the fact that those couldn't be correct. This was to hopefully cover up the extend from the authorities. (Needless to say, that they didn't succeed.)
And for a third example, it was days of constant radiation before the authorities decided to evacuate the people of Prypjat. And even then told them it was just a safety measure and they would only be gone for a few days. The latter surely contributed to working against a panic. But the time they took before deciding on evacuation is crazy!
Knowing that, it's completely plausible, that A the investigation of the DPI was just fucked up, B there were some people on the lower end of the "chain of command" who wanted to safe their own asses or C the government itself wanted to keep something hidden.
All in all I find B the most fitting, but that's just my personal opinion.
GlennM, there was a plan, but it was not transmitted to the committees as should have been. Yuri Yudin knew, that they expected to be longer, because Igor told him to pass that message on. Yudin forgot to do so. But when he came back (after some time with his family), he was surprised, that the group was not back.
The lack of a report that someone was following the group is no evidence, I'd say. If there was an order, they would have kept it secret. And if it was a good military or paramilitary group, they would not let their "prey" know of their presence.
It seems I really have to get the book by Eichar...
As for the cosmonauts: I don't believe there is a way to have secret space flights. The start may be kept secret. But at some point, the rocket would have been detected. Therefore I'd expect the flights to have failed end thus killed the pilots. Am I wrong? Do you have a link with that information? I never heard of that.