@Stine-Kaastrup0 And your answers:
1) Clearly this shouldn't be possible. Remember when blocking a date it is the night you are blocking not the day. Also if you have linked the calendar to another listing blocking one will not block the other. Other than that it is an Airbnb glitch or, if I am honest, more likely user error on your part.
When you block a date make sure it is blocked by refreshing and looking at your calendar.
Yes you will get fined if you cancel the booking
Never cancel for a guest - explain to them that you will get fined and the dates will be blocked so you cannot get a replacement booking. They need to cancel and then ask Airbnb for a refund - Airbnb will then contact you and see if you are willing to refund.
3) Just go to your calendar, highlight a night or nights and type in a new rate per night.
Hope this helps