Scrum requires no development practices, but good development practices do help and make scrum more successful.

Detailed Explanation: A Sprint can be canceled if the Sprint Goal becomes obsolete, as stated in the Scrum Guide. This decision is made by the Product Owner, and it emphasizes Scrum's focus on delivering value.

Changing the team members is sometimes necessary, this will have an effect on productivity. So it should only be used when really necessary.

The scrum guide says: "The Developers who will be doing the work are responsible for the sizing. The Product Owner may influence the Developers by helping them understand and select trade-offs."

The design grows as more is learnt.

All of these would good moments to update the PO with the work done.

It is not fixed, it is a live artifact that will grow and change as more is learnt. The product owner orders it by value.