For your question on the base ya it is a full CentOS core install so you can install do whatever you want.
Stability: CentOS and even the elrepo kernel are pretty stable but both BTRFS and Rockstor are both under fairly heavy development so I would plan updates around that and apply manually for now as well as getting the stable branch subscription. Plus if you do go the USB route make yourself a master img then a bad update is a reimg to fix.
Cloud backup has been discussed in the Rock on requests thread and replication to another rockstor box using btrfs send is built in.
Spotlight: pretty sure this is just the mac search so you shouldn’t have any trouble mounting the drive and then searching it works for windows.
Your /fd0 error is most likely a flobby drive error so for whatever reason you vm thinks there is or was a floppy drive no biggie.