Effects of "Full-Copy the files"
I implied but failed to specify that the "Backup Method for Files" selected on the GENERAL tab was "Full-Copy the files" for the occasional USB jobs. To test this I ran the offsite job, ran the policy Daily Diff job, disconnected the USB rive, and restored some files from the Differential. To my surprise the files restored fine, and timestamps on the B2D files and IMG folders show that the DD looked back past the Full Offsite job to the previous successful scheduled Weekly Full backup. The restored files existed in the WF and were not changed for the DD. Undocumented feature of the AVVI?
Ideally, the backup method which is data-specific will be applied, i.e the backup method chosen in the VMware tab settings will apply. This is beneficial when you have multiple types of data to be backed up in the same backup definition. I am not really sure what is the impact when you have just one type of data to backup, but have specified different backup methods in the same backup definition.
The section titled "Configuring backup methods for backup jobs" of the BE 2014 admin guide may be the documentation which you are looking for.