Releasing the product at the end of the sprint doesn't relate to one item and when it's done. Code coverage does, but it's the worst criteria in this list, because it's a stimulus to write tests that cover a lot of lines rather than cover the functionality.

If you are not sure, review the daily scrum section in the scrum guide.

Never should they reduce the DoD. They should be able to do the work their selves and therefore work with the product owner to reduce the scope. Since the product owner is responsible for maximizing value and making priorities, he is the go to person for matteres like this.

The test gets written first in TDD, after that, the production code will be written to satisfy the test.

Architecture emerges as more is created. There are often guidelines and principles in place.