Can an Active Directory Domain be restored from a RODC (Read-Only Domain Controller) with a copy of the GC (Global Catalog)? Does an offsite DC replicating the AD need to be a full domain controller to provide a sufficient up to date AD Backups for disaster recovery?
Your RODC does not contain a full copy of AD, it is missing things like security credentials, encryption keys etc. Full domain controllers are also set not to replicate changes from (pull) RODC's, because the database at the RODC should not change (aside from when it is updated by a full DC) so any changes that are made are likely to be malicious and should not be replicated.
Because of these restrictions, you are really going to want to use another full DC if your aim is to provide some sort of redundancy for your DC. Not this is redundancy, its not a backup.