My background investigation was closed in August but I still could not ship to BCT due to sudden changes in regulations. You are not right, there are changes implemented by DoD or other higher officials and delay is not purely due OPM investigations backlog.

how long after biometrics did you receive interview invite letter?

I submitted everything together at the local USCIS. I gave them my work authorization form together with my DA application. Will they process it right after or I will have to apply for the work permit separately. Do they process work permits at local USCIS offices?

What is the timeline? How long will it take to receive a work permit?

Why do you think there is an interview?