Ok, so here's what I'm getting. Every back up would be set for one persons phone only do to the fact that each phone provides its own key locked to that file. I'm assuming then the idea of dev unlocking would be the only way to get a universal file since it would be a "testing" version.
As far as the sd card goes, using anything outside of dell would provide an sd card for that phone. So, no, the samsung idea wouldn't work though that would be a good theory since they're all windows phones. From what I gather, that would mean anything outside of your own phone would produce unwanted files and not a backup for you to use since the key for it would only be decrypted by the phone that created it.
I'm learning very quickly of 2 things that are being made very obviousvery quickly.
1. Dell sucks.
2. Windows phones suck as well.
(1 & 2 refer to the blocking of modding)
I'm guessing then that each DVP creates a whitelist to block all other phones that are not it(?) That sounds a bit confusing.
These are all inferences, nothing is set in stone or absolute. I could be misreading some of these and clafirication would be nice.