While a backup is running for Confluence, will I still be able to access Confluence, and all data therein?
Assuming you mean the Scheduled Backups or manual xml export: yes, your Confluence will be available. However you might notice some performance issues during the backup.
For production backups Atlassian recommends database and home direcotry backups instead: https://confluence.atlassian.com/doc/production-backup-strategy-38797389.html
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