It’s odd behaviour but your options are in your favour
They are liable for the whole AST
Whether they will pay once they leave is another matter and it depends upon how far you go to collect it
You may want ask for the first month and cover your cost of cleaning and advertising and the cost of Open rent collecting rent etc
You can ask up to the cost of rent for the AST that you would have earnt
I would sign a deed of surrender so its formalised
Regarding viewings you could do digital viewings. It is in their favour if you find someone sooner rather than later so they don’t have to pay rent