Although they got your file by the 1st interview schedule as you said...It took them 4 to 5 month to reschedule...that is way too long...
Yeah, I kinda thought it was a little long too. It beats getting stuck in name check for 2 years, but it still sucks. You are right, however, it seems to be completely in the hands of the DO once a descheduling happens.
It's taken the Dallas DO nearly 13 months to process my daughter's N-600 (it should take about 15 minutes to process an N-600, only the USCIS could figure out how to spend a year and $255 doing it). Hopefully that will be over this wek. I'm not a big USCIS fan.
Good luck.