Believe it or not, you're right on the money with everything you've written.
CommServe LiveSync provides test/maintenance/production cutover functionality, and is typically run ever 15 minutes. There can be multiple passive CommServes, and you could put in AWS if you want.
Yes, the MediaAgent is separate and the Indexes in Commvault are distributed in the CommServe Database (CommServe), Deduplication Database (MediaAgents), and the metadata/content index of the Backup Jobs. In the event of losing a Deduplication Database, you can still restore backups from the Backup Library because the indexes in the deduplication database are only needed for backup and not restore. The Passive CommServe can recover and continue backups to the default Storage Polocy Copies. Commvault Storage Policies allow for secondary copies to be promoted as primary, so if you conclude that your Datacenter is going to be fubar for extended period of time - the AWS CommServe and AWS MediaAgent can be configured to continue with backups and restores.