XLSForm Online Preview link validity

Yes, ODK Central can be used in production! ODK Central has been out of beta for the past couple of releases.

See this help article for information about permissions in ODK Aggregate. Note that a user's permissions in Aggregate are the same for all forms: a user cannot have different permissions to different forms. See this forum discussion for more information.

Note that Central allows permissions to be managed at a more granular level: forms and users are grouped by Project, and you can also manage form access within a Project.

I'm not sure about this, but I would recommend creating a separate topic about this question.