In history pages, add combo boxes with listed important dates (milestones, fcs, cut-off) from other versions. That's easy part.

If comparing older version vs "some version", list of rows and columns is created from submitted version. Snapshots contains information only about issues and component_id, against which the issue was reported at the time of the snapshot.

This means, that current implementation is not expecting any conflicts (different rows/column in each record). It simply loads data from dateA, creates table and then matches data from dateB to the table.