Hey Phi, the heads work just like the mythical Hydra heads which I don't remember if they ever merge back into one. I do however remember that the hydra could not be killed by cutting off its heads. Unlike the real hydra, Hydragon can be killed by attacking her body, so not quite immortal but attacking the heads do nothing.

As for the clansmen, they are NPCs, they will actually fight in quest threads but will most likely only observe in citadel threads.