Thanks, ATS.
I sit at the back - except when travelling to Madrid - usually in row 32 (or 33 now that my hand luggage can be stored under the seat), and have noticed rear rows blocked off when aircraft seem to be as full as you describe.
As far as flight deck crew allowing passengers to occupy blocked off rows is concerned, I've seen this twice. On one occasion, a flight attendant went forward to the flight deck and returned with permission; on the second, a member of the flight deck crew actually came aft.