I am a little confused:
"need to be able to access a backup copy of my AC Database on a machine different than my main server"
Do you mean restore to a different machine from the server that is down?
And, what is wrong with restoring it as is? It was fine then? And why an older backup? I am not a SQL expert, but when in SSMS, the tables are usually read only anyway.
How are you going to stop someone adding addenda? You can add addenda to any AC patient. Not sure if I can help, but can you clarify? And, I am not sure where AC is with these .enc backups? Can you not use one of those?