<I would have thought the data would automatically revert to the DOJ>
I think that probably is what happens -- but we're talking about a rogue, criminal president.
<I was thinking more of Mueller or his staff deciding to save a backup, illegally if necessary, just long enough for the Congress to begin impeachment process>
I doubt Mueller would do anything illegal, but I think he would and will find a legal way to preserve evidence.
Lock him up.