1. My Bitbucket Server instance is used under a Community/OpenSource/Classroom license. Can I get File Viewer for free?

Yes, in this case you can request a free Community/OpenSource/Classroom license for File Viewer directly from Atlassian. To do that, please, follow this link.

Your free File Viewer license is valid only if your organization is approved by Atlassian for a Community/OpenSource/Classroom license type. To learn more about Atlassian Community, OpenSource, Classroom licenses, see this page.