You are missing the weight force. The wing should stall first because then it will produce less lift and the weight will make the aircraft pitch down.
In attached flow, the lift from wing and tail is balanced such that the combined resulting force is acting exactly at the longitudinal position of the center of gravity. If the wing stalls, the balance of lift is shifted backwards (regardless of the tail producing lift or a downforce), because now the forward part of the wing-tail-combination will produce proportionally less lift than it did in attached flow. The center of gravity will then be ahead of the resulting lift force and will pull the nose of the aircraft down.
A tail stall is bad news for the pilot and should be avoided:
- A stalled tail will have much less elevator effectivity, reducing the control power available to the pilot
- A stalled tail at high angle of attack is stalling at maximum lift, so it produced positive lift before. If it stalls, its lift will shrink and make the aircraft pitch up.
In a conventional configuration the tail flies in the flow field of the wing. Because the wing's downwash increases with angle of attack, the angle of attack variations at the tail are reduced. This helps to keep the flow at the tail attached over the whole useable angle of attack range of the wing. A tail stall is either caused by a wrong location of the center of gravity (CG) or by a very poor transsonic design which causes shock-induced stalling of the tail when the wing is still doing fine. Very rarely can you stall the tail by a wrong trim setting when a moveable stabilizer is used for trimming, or in a deep stall.
Some supersonic aircraft compensate for the backward shift of the center of lift in supersonic flow by pumping fuel from forward to rear. If an airplane with such a rear CG location slows down to subsonic speed, it will need to produce proportionally more lift with the tail than with the wing. If it now pitches up (say, for a tight turn) it will risk a tail stall. Note that such a configuration is aerodynamically unstable at subsonic speed.
EDIT:
Now you define the center of gravity as ahead of the wing, which makes for a very stable configuration. The tail is producing a downforce to compensate the pitch-down action of the wing's lift.
If the wing stalls first, lift will become less than weight and the aircraft will accelerate downwards. Now the angle of attack will increase at both the tail and the wing; at the tail even for two reasons:
- The downward acceleration will change the local flow direction to a larger angle of attack, and
- The diminished downwash from the wing will cause an angle of attack increase of its own at the tail. This effect takes a little longer to manifest itself: The delay is the distance between the quarter points of tail and wing divided by the airspeed.
Both effects will quickly create a dominant pitch-down moment for the aircraft because the tail will contribute much less downforce than before the stall, and the diminished pitch-down moment of the wing will become insignificant.
