Host cancelled two of my reservations (different flat) stating that 'She want to wait for guests with longer stays'. Is it a valid cancellation reason?
@Mateusz16 No, it's not. And this host sounds really clueless about the penalties she'll receive for cancelling- the dates she cancelled your reservations for will be blocked by Airbnb so she won't be able to rebook them for the longer stays she's waiting for.
You're better off not dealing with a host like this. You could report her to Airbnb for the invalid reason she gave for your cancellation.
Absolutely not as stated above. The penalties incurred by the host are simply not worth it. If they're looking for longterm stays they need to make that adjustment in the Listing --> Availability --> Trip Length --> Minimum Stay section. Doing so however would most likely limit a host's reservations.