One last hint about the final city EA served from DTW in 1977: EA's Jan 10, 1976 timetable says EA flew nonstop from the missing city to 23 other cities. However, none of EA's flights out of the missing city were on L-1011 Whisperliners. Although DL was much smaller than EA at the missing city, DL did serve this city with L-1011s on a regular basis in the 1970s.

I'll close this question out this afternoon if nobody answers it correctly.