Were they told of the downgrade at boarding? Or were they at least called up to the podium in advance? What was said to them at the time?

Did the GA give them decent seats together, ideally in EC/C+?

Is the OP a DL elite or at least a frequent traveler? My instinct on noticing the aircraft down gauge would probably have been to call the DM line and ask whether FC was now overbooked, even though the seat assignments appeared intact. In addition to watching the seat map, I would have called DL periodically as the day of departure approached to ask specifically whether the flight was overbooked in FC. Depending on the answers, I might have explored alternative routings, although I say this partly because I consider the CRJ-900 much better than a CRJ-700.

I agree with the suggestions above that it would have been better to delete the last paragraph of the complaint. Also, I would expect DL to do more for elites (or for passengers on award tickets from elite DL FF accounts) in these circumstances.