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 scrum guide says: "The Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team." The key here is the value maximizing part. Of course, he also comes up with requirements and acts as a go-between between developers and customers, but that is definitely not the best answer here.

A ready product item needs to be able to be done in a sprint (scrum guide). In order for it to be ready, the product owner and the developers need to understand it well. Tracking tools and requirements analysis and how user stories should be made isn't part of scrum or the best practices for development.

The product owner can cancel the sprint if the sprint goal becomes obsolete. This is a very rare event. It can happen when a company losing a client or the client decides to take a completely different turn.