Why does the use of reverse thrust on the DC-8 generate a pitch-up moment rather than a pitch-down moment?
What you're missing from your diagram is @JohnK's drawing (which I'll borrow) from his answer to your related question, Why does the use of reverse thrust in flight on the DC-8 cause areas of separated/reversed airflow over the wing?



The use of reverse thrust increases the wing's angle of attack (AOA) more than it does for the tailplane (as opposed to if the whole plane had pitched up). And not only that, the inboard thicker part of the wing now starts with turbulent air, this reduces the amount of lift at the neutral point (aka aerodynamic center), unlike what happens in a normal AOA increase.

Since the AOA of the tailplane hasn't changed much, it can't contribute to the stability, and with the big reduction in lift, the tailplane wins and the plane pitches up.



Think of it as deploying spoilers, but spoilers that affect the wing from before the leading edge:


  When extended, wing-mounted speed brakes usually produce a nose-up pitching moment (Boeing).