The pairs of seats in coach are not necessarily blocked. In fact, in some cases they show as blocked because they're being held for passengers with disabilities, service or emotional support animals, etc. and can easily be unblocked by DL employees. I've been offered such seats in the past by DL phone agents just because there were no other decent preferred seats available on the flight.

IMO the GA shouldn't have assigned such a seat to a non rev if there were other unblocked open seats, although the normal rule is that non revs get the best seat available (in priority order), so I don't think this would officially violate the rules. I've also noticed that some GAs tend to assign seats from front to rear with no further thought (but not to genuine middle seats now unless it's a group traveling together), so that the first person on the upgrade list for FC can get the least desirable (bulkhead) seat.

The seat would be a natural choice for a working FA to occupy for takeoff and landing, although I don't like the idea of a FA sitting next to someone for significant rest periods during flight (but again this probably doesn't violate rules for a pair of coach seats).

You might be able to get DL to either refund the extra seat reservation fee that you paid for such a special seat or give some customer service gesture (a small number of miles).

Once again, was the FA on duty, NRPS, or NRSA? And did you say anything about the seating during the flight? Was the FA wearing a mask properly at all times, covering both mouth and nose?