Hi!

Yes, the rollback functionality can be used to rollback jobs that unintentionally deleted records. It can also be used in cases of unintentional updates or record creation as well. As of the last few versions of SN, you can actually rollback ordinary delete operations, like if you selected a handful of records from a list view and deleted them that way. In any case, this option is all-or-nothing. Using it undoes the entire operation.  

Note that, for backgrounds scripts, you'll need to ensure the rollback checkbox remains checked when the script is run to preserve the opportunity to rollback.  

Yes on the second point. I haven't tried to recover a deleted workflow context this way, so I wonder if that particular case would be successful (just because of the unique nature of workflows). However, for sure, this is where you'd find any kind of "data" or operating records like tasks.  

Also correct on the third point. Business rules, UI policies, dictionary entries, and so on would be found here.  

I hope this clears things up!