Sorry, but you seem to have some big misconceptions about Scrum. It's totally false that changes are not allowed during a Sprint. Lots of people associated with the Scrum Guide went at great lengths to clarify this yet this misconception lives. In Scrum you negotiate changes during the Sprint, between PO and Dev team, usually down to replacing Sprint items with others, stopping ongoing work, etc.
Also, communication with customers/external stakeholders isn't done through Scrum Master, it's vdone through Product Owner who can decide whether new items and changes go to backlog or negotiates introducing them in the current Sprint.