Hello,
The disk image can be backed up no matter it is locked or unlocked. There is a slight chance for invalid backup if the data is written to the folder at the very same moment the Backup is saving the image, but unless you use the folder for some continuously writing purpose, you should be fine.
The V3 does support custom location of the disk image, so in case you are using different location then the default Application Support/Espionage/Data, then make sure you backup that location too.
Re unlocking the image without Espionage, yes the possibility exists but you need to take some steps up front. The V3 automatically generates the password for each disk image. You can get this password for each folder by clicking on retrieve password button. So you should basically copy all the password out and store them in some super safe place, like some password management app.
I hope this answers you question(s)