You can't, we are sorry. At the moment importing data into an existing collection is not supported.

You can't move a document to another collection, we are sorry. Each collection is a separate entity.

Yes, you can add multiple pictures with the Group field. Add a Group field and select Picture. Also, the Group field allows multiple values for any other field type.

Yes, you can export a full backup from Settings → Export → Backup archive.

Data is safely stored on iCloud using the CloudKit framework. A local cache of the database is stored on device in a private folder.

You can't change location of the database or move it to a cloud service, we are sorry.