The helicopter crash that is depicted in the series did occur. However, it didn't happen when it is shown in the show. In the series, it happens as they are dumping the sand and boron into the reactor while the fire is still burning. In reality, the crash occurred much later in the recovery process. The helicopter actually crashed near the end of the installation of the initial containment sarcophagus over building 4 (which is not shown in the show) The helicopter was doing a final inspection and the rotors hit an obstacle, causing it to crash.
People refer to each other in the form "Comrade-surname," which is inappropriate among colleagues. Dyatlov's subordinates would have called him "Dyatlov" among themselves and "Anatoly Stepanovich" (his first name and patronymic) when addressing him directly, rather than "Comrade Dyatlov." However, it is likely the writers decided against using the correct forms of address to avoid confusion with non-Russian viewers, who might think, for example, that Stepanovich is Dyatlov's surname.